A study was carried out in 16 patients with moderately severe hypertension to investigate the effects of nifedipine, given alone or combined with a diuretic, on blood pressure and on renal and platelet function. After 4 weeks on placebo, patients were randomized to receive treatment for 6 weeks with either 20 mg, nifedipine twice daily or 25 mg mefruside once daily on a double-blind, double-dummy basis. All patients then received treatment for a further 6 weeks with a combination of the two drugs in the same dosage as before. The results of blood pressure measurements and laboratory investigations during the three phases of the study showed that significantly better blood pressure control was achieved with nifedipine alone than with mefruside alone. Mefruside had an additional hypotensive effect when added to nifedipine. There was no significant change in the renal blood flow or glomerular filtration rate, with a satisfactory control of blood pressure. There was also no detectable change in platelet aggregation with increasing concentrations of ADP and ristocetin. An adaptive mechanism could be responsible for the apparent lack of change compared with single dose studies.