The action follows a renewed Eta campaign to extort "revolutionary taxes" from the region's industrialists.
Cosme Delclaux, 34, son of a prosperous glass-making family linked to a powerful Basque bank, disappeared last Monday in Las Arenas near Bilbao. His car was found near his workplace two days later. Police believe Eta will demand a huge ransom and use the crime to frighten other Basque entrepreneurs.
Extortion of "revolutionary taxes" is Eta's favoured method of raising funds for its fight for an independent Basque homeland, and this is the third such campaign in a year. Some 50 businessmen and professionals have received threatening letters in the past month, demanding up to pounds 30,000 for the "liberation process" and offering to accept payment in instalments.
The Interior Minister, Jaime Mayor Oreja, admitted the recent lull in Eta activity was no more than "a mirage". Police think the latest action indicates the recomposition of an Eta strike force.
The government has appealed for recipients not to yield to threatening letters but to tell the police, but many will be too frightened to do so. A businessmen's leader said the kidnapping "worsened an already intolerable situation" by heightening the climate of fear and threatening to derail measures to bolster the region's flagging industrial activity.
Eta already holds hostage Jose Antonio Ortega Lara, a prison officer seized more than 10 months ago. Last April Eta released the businessman Jose Maria Aldaya, reportedly on payment of more than pounds 500,000, after nearly a year.
The latest action has ended cautious moves to bring nearer home some of the 540 Eta prisoners held throughout Spain, meeting one of Eta's principal demands. That process was the fruit of talks between democratic Basque nationalists and the interior ministry. Basque leaders do not deny the existence of contacts with people close to Eta, and all parties see the need for political as well as policing measures to end the guerrilla war.
If confirmed as an Eta operation, the kidnapping marks the 76th in a campaign that has claimed more than 800 lives in 30 years.