Zofenopril calcium (CAS 81938-43-4) is a new angiotensin converting enzyme (ACE) inhibitor, which in addition to the typical activity of the class, proved to possess a specific cardioprotective effect due also to the presence of the sulfhydryl group. In this trial zofenopril calcium and enalapril maleate (CAS 76095-16-4) were given to 20 healthy volunteers of both sexes in repeated dose regiment at two dose levels: 30 mg and 60 mg zofenopril calcium and 10 mg and 20 mg enalapril maleate. The study was conducted according to a two-period, two-sequence, crossover design, with washout. ACE activity in serum and zofenopril, zofenoprilat, enalapril and enalaprilat plasma concentrations were determined during and on the last day of the two study periods. Both zofenopril and enalapril were extensively converted through hydrolysis to their active metabolites zofenoprilat and enalaprilat, respectively. Zofenopril exhibited a complete and a more rapid hydrolysis rate compared to enalapril, which is reflected by the higher metabolite to parent drug ratio of Cmax and AUCss, tau showed by this compound. Even though only two dose levels were investigated in this trial, the pharmacokinetics of both drugs seem to be linear. In line with previous trials, both compounds at both dose levels investigated produced complete or almost complete inhibition of ACE activity in serum, for a period lasting 6-8 h after administration, the inhibition being still relevant 24 h thereafter. The tolerability of the two drugs at both dose levels proved to be very good as demonstrated by subjective and objective symptoms, by the absence of relevant adverse events, and by laboratory biochemical parameters and vital signs evaluated before and after the trial. Blood pressure showed a fairly decreasing trend with both the drugs, systolic and diastolic blood pressure values being however within normal range in all the subjects. In no case symptoms of hypotension were experienced. In conclusion, zofenopril calcium and enalapril maleate show very good tolerability and appear to exert similar activity on serum ACE. The main difference in the pharmacokinetics of the two compounds is the conversion from pro-drug to the active metabolite which is faster with zofenopril.