Thirty patients (21 F, 9 M) of mean age 55.3 years with non-insulin dependent diabetes mellitus of mean duration 10.8 years and hypertension (blood pressure 160/95 mm Hg) of 0.3 to 4.0 years were randomly allocated to either a twice daily regimen of captopril (50 mg twice daily, Group A) or a once daily captopril schedule (50 mg once daily, Group B). Good glycaemic control (HbA1c, mean 7.63%) had been achieved with sulphonylureas in 22 patients and insulin in 8 patients. There were no statistical differences in baseline values between the two groups. For the 15 patients in Group A, blood pressure fell significantly from a baseline of 177 +/- 13.2/106 +/- 7.8 mm Hg to 161 +/- 14.3/91 +/- 6.9 after one month (P less than 0.05) and continued to decrease at 3 and 6 months. In Group B the blood pressure changed from 179 +/- 19.0/110 +/- 15.6 to 169 +/- 21.0/98 +/- 7.2 at 1 month (P less than 0.05) with further reductions again seen at 3 and 6 months. Nine patients had poorer response than the other 21 but there were no demographic differences between these subgroups nor were there any differences in plasma renin activity or aldosterone responses. There were no statistically significant changes in haematological or biochemical values in either group during treatment. In particular, HbA1c and fasting glucose were unaffected by captopril treatment. No side effects were encountered during the 6 months of follow-up. In conclusion, captopril is an effective antihypertensive in non-insulin dependent diabetes mellitus with mild to moderate hypertension and a once daily regimen could improve compliance.