Based on pooled data from three randomized placebo-controlled dose-finding studies in a total of 489 patients, the dose-response relationship for efficacy and adverse events was estimated, using the Michaelis-Menten equation: Effect = maximal effect multiplied by dose/constant plus dose. Three conclusions were derived from the pooled data: (1) A marked increase in efficacy is seen when the reduction in diastolic blood pressure after one week of treatment is compared with that seen after five weeks of treatment, with both placebo and active treatment. Thus, dose increases should preferably be made at intervals of at least four weeks to avoid unnecessarily high doses. (2) Isradipine 2.5 mg twice daily offers an efficacy of approximately 80 percent of the maximum with an incidence of adverse events which, statistically, is not significantly different from the incidence seen in the placebo groups. (3) With continued treatment, a marked decrease in the incidence of adverse drug reactions is seen between the first and fifth weeks, especially with doses at 1.25 and 2.5 mg twice daily. However, with doses above 10 mg per day, this effect is no longer evident.