Triacsin A, 1-hydroxy-3-(E,E-2',4'-undecadienylidine) triazene and triacsin C, 1-hydroxy-3-(E,E,E-2',4',7'-undecatrienylidine) triazene are potent inhibitors of acyl-CoA synthetase (EC 126.96.36.199). The concentrations of triacsin A required for 50% inhibition of acyl-CoA synthetase from Pseudomonas aeruginosa and from rat liver are 17 and 18 microM, and those of triacsin C are 3.6 and 8.7 microM, respectively. Kinetic analysis indicates that inhibition of triacsin A is non-competitive with respect to the two substrates ATP and coenzyme A, but is competitive with respect to long-chain fatty acids. The apparent Ki value is 8.97 microM when oleic acid is used as substrate. Acid hydrolysis of triacsins results in corresponding polyenic aldehydes with no activity. This suggests that the N-hydroxytriazene moiety is essential for inhibitory activity against acyl-CoA synthetase.