The antihypertensive efficacy of monotherapy with the calcium antagonists nifedipine (n = 60) and verapamil (n = 43) was investigated in patients with essential hypertension. Relationships between age, pretreatment blood pressure, pretreatment plasma renin activity, and hypotensive response were examined. Intraindividual differences in the responses to both drugs were also studied (n = 16). Treatment with verapamil had to be discontinued because of constipation in one patient; treatment with nifedipine was discontinued because of headache in 11 and ankle edema in one. The antihypertensive efficacy of nifedipine and verapamil was comparable and was positively related to pretreatment mean blood pressure (p less than 0.001) and inversely to pretreatment plasma renin activity (p less than 0.05). With verapamil, the fall in blood pressure correlated positively with the patient's age (p less than 0.001). Thus, calcium antagonists can be used as first-line drugs, especially in older and low renin patients. The relationship between the antihypertensive response and pretreatment blood pressure suggests that there may be a calcium influx-dependent vasoconstrictor mechanism in essential hypertension which may be more pronounced in patients with low plasma renin.