Free radical (FR) scavenging may be a therapeutically useful adjunctive property of angiotensin converting enzyme (ACE) inhibitors. In this study we have shown that SH-containing ACE inhibitors (captopril, epicaptopril, zofenopril) are potent FR scavengers at a concentration of 4 x 10(-5) M whereas non-SH ACE inhibitors (enalaprilat, quinaprilat and perindoprilat) have no FR-scavenging activity at this concentration. Furthermore, the SH-containing agents preferentially scavenged general radicals rather than superoxide radicals, i.e. suggesting that these drugs would be effective in quenching the culprit FR in ischaemia/reperfusion injury.