During car journeys the 38-year- old peer, once estimated to be worth pounds 20m, would stop at service stations and cafes and within a few minutes would change from being 'subdued and lethargic' to an 'excitable and alert' state, Christopher Ball QC, for the prosecution, said.
Mr Ball told Snaresbrook Crown Court, east London, that the peer regularly supplied his constant companion, James Whitby, with the class A substances whenever his friend said he wanted to 'freshen up'. During a shoot at his home at Ickworth House, near Bury St Edmunds, Suffolk, he had also given cocaine to a second acquaintance.
The marquess, charged in the name of Frederick John Hervey, denies two charges of supplying cocaine and one of supplying heroin between September 1990 and October 1991.
Mr Ball said the alleged offences came to light after one of his business acquaintances, Bruce Smith, who was helping to develop plans for a motor museum at Ickworth House, fell out with him and told police about the drug activities.
The court heard that during one particular car journey with Mr Smith, the marquess told him to pull over and explained why they had to keep stopping the car. 'He removed from his jacket a green leather pouch, put it on his lap, opened it and took out two bottles, one containing white powder and the other brown,' Mr Ball said.
The defendant poured a quantity of both on to the car's centre console, 'took a banknote, rolled it up and snorted the drugs. He explained the white powder was cocaine, which 'gets me going, I don't function without it'. Of the brown powder, heroin, he said, 'This slows me down a bit, makes me more mellow'.'
The court would also be told of a container that the marquess allegedly used to secrete his drugs. 'It is a Pledge polish can, Johnson Lemon Wax, wax every time you dust. It is not quite the sort of can you would be buying in the Co-op, because this can, you will see, had a false bottom, a bottom that screws in and screws out and had a padded lining,' Mr Ball said.
The case continues today.