The effects of the calcium antagonist, felodipine, and hydrochlorothiazide (HCTZ) on natriuresis/diuresis and blood pressure were evaluated in 12 healthy subjects. The investigation was designed as a double-blind, three-way, randomised, crossover study, and all comparisons were performed against placebo. Urine volume, urine sodium excretion, heart rate and blood pressure were measured after a single dose of felodipine 10 mg, HCTZ 12.5 mg or placebo as well as during steady-state conditions (6 days of treatment with felodipine 10 mg b.i.d., HCTZ 12.5 mg b.i.d. or placebo). A significant increase in natriuresis was seen in the first 4 h after a single dose of felodipine and HCTZ, and the effect of felodipine was approximately 40% that of HCTZ. When the entire 24-h period after a single dose was studied, there was a significant increase in natriuresis after HCTZ, but not after felodipine, compared with placebo. A significant increase in diuresis was found in the first 4 h after a single dose of HCTZ, but not after felodipine, compared with placebo. Under steady-state conditions, there were no statistically significant differences between felodipine and placebo or HCTZ and placebo when the 24-h period, as a whole was considered. Potassium excretion was not affected by any of the drugs. Felodipine caused a significant decrease in diastolic blood pressure in this study. This was not the case for HCTZ or placebo.