The free radical scavenging effects of an angiotensin-converting enzyme (ACE) inhibitor containing sulfhydryl (SH; captopril) were compared with those of ACE inhibitors not containing SH (enalapril, enalaprilat, delapril and its de-esterified products). Electron spin resonance (ESR) using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trap showed that enalapril and delapril (0.6-4.8 mmol/l) inhibited hydroxyl radicals concentration-dependently. Captopril (2.4-10 mmol/l), enalaprilat and the de-esterified product of delapril (1.2-4.8 mmol/l) also inhibited hydroxyl radicals in a concentration-dependent manner. The effects of captopril and enalapril (2.4-4.8 mmol/l) on the scavenging of superoxide anion radical were also concentration-dependent. Delapril, its de-esterified product, and enalaprilat weakly inhibited superoxide anion radical. These results indicate that both SH- and non-SH-containing ACE inhibitors scavenge hydroxyl radical more strongly than the superoxide anion radical and that the free radical scavenging action of ACE inhibitors is probably not related only to the presence of the SH radical.