At one time the shares were up nearly 140p, with market-makers allowed to shelter behind "indicative" prices for nearly an hour.
Then, prodded by the Stock Exchange, the drugs group attempted to quell the speculation by saying it knew of no reason for the strength of its shares "other than possible investor assessment of the strength of its new product portfolio".
Such a comment reduced the heat but with many market men convinced a bid is about to materialise the shares finished 74.5p higher at 1,068p, a peak.
Hoffman La Roche, the Swiss group, remains the favourite to strike. London's two Swiss-owned securities houses, UBS and Swiss Bank Corporation, appeared to be active players in Zeneca, helping promote the Hoffman theory.
Since the start of last week Zeneca shares have soared 137p in often active trading. Ramps come in all shapes and sizes but if Zeneca is an example of old-fashioned market spivery, then an awful lot of money has been invested in the deception and many seemingly not-so-smart punters are experiencing the pain of burnt fingers.
More than 7 million shares were printed yesterday, representing the busiest day's trading since Zeneca was split from ICI two years ago.
Trading in the drug group's shares has in recent days been brisk, rather than spectacular. And market men have drawn attention to the apparent leaks that preceded earlier Swiss bids; the latest being the Compagnie Financiere Richemont bid for Rothmans International.
Hoffman refused comment on the rumours; Sandoz, another Swiss group sheltering behind a "no comment", was in the frame. Bristol Myers Squibb carried the flag for the US.
At the close Zeneca was valued at pounds 10bn. Its position has looked vulnerable since it failed to merge with Wellcome, which fell into the arms of Glaxo.
There have been bullish reviews of Zeneca in the past week but it is the group's seeming need to join with another big drug producer that has provoked the buying rush. Indeed, the case for drug groups indulging in defensive mergers has a wide following and helped create the impression that Zeneca could descend on Fisons, which is involved in protracted take- over talks with Medeva. A three-way Zeneca-Fisons-Medeva merger is even a thought occupying many market minds. Fisons gained 11p at 193p and Medeva 1p at 249p.
SmithKline Beecham, seemingly sitting quietly on the sidelines as the drug fever has raged, rose 16.5p to 551.5p. As excitement has swirled around Zeneca SB shares, helped by US support, have climbed more than 50p. Glaxo Wellcome, reflecting its involvement in a Canadian group that has produced an Aids treatment, gained 16.5p to 749p.
The Zeneca escapade overshadowed the day's other big event - the pounds 835m GEC shot for VSEL. The blockbuster offer pushed VSEL 302p higher to 2,140p and seemingly relegated British Aerospace, 14p lower at 527p, to the status of also-ran.
The market had a firm session, with the FT-SE 100 index up 10 points (after 24.2) at 3,380.8. The possibility, still regarded as remote, of lower interest rates and a firm New York display helped sentiment. But government stocks failed to offer any encouragement, suffering falls approaching pounds 1.50.
In a weak insurance market Commercial Union dipped 18p to 615p; the group is due today to take analysts to Paris to discuss its Groupe Victorie off-shoot, taken over last year.
Great Universal Stores lost 9.5p to 624p, with Cazenove said to be negative, and Wickes had a subdued reception after meeting analysts, falling 8p to 114p.
Etam, warning of a first-half loss, tumbled 30p to 176p and Austin Reed lost 14p to 198p following a downbeat shareholders' meeting.
WH Smith, on the departure of its retail director, Peter Broughton, slipped 3p to 352p.
Dividend worries tormented NFC, the old National Freight Corporation, off 8p at 169p. Figures are due next week. Hard-pressed MR Data Management edged ahead 3p to 63p as Colin Haycock moved in as chairman. He was formerly with WS Atkins, the engineering consultants.
Pilkington, the glass group, jumped 11p to 189p on the surprise dividend increase and Rolls-Royce put on 4.5p to 190.5p as Lehman Brothers put out a buy recommendation. A return to profits lifted 600 Group, the scrap group, 13p to 94p.
TeleWest, the country's largest cable television group, rose 5.5p to 169p on its takeover of SBC CableComms, the fifth-largest operator. Nynex, the cable TV group coming to market, is expected to be priced at 135p.
Navan Resources jumped 21p to 185p as Homestake, a US group, agreed to pump in $24m for a 10.25 per cent stake.
John Mansfield, the timber shell, edged ahead 0.25p to 3.75p as Alan Sharman sold and director Rodney Graves went to 8.8 per cent.Reuse content