A triphasic pattern of blood pressure response was observed in 13 of 26 patients treated with captopril. Pressure fell after the first dose in all but 3 patients, reaching a minimum by 90 min. Over the next several days, pressure either remained at this level (n = 13) or returned towards control levels (n = 13) even with increased dosage. Blood pressure then fell gradually, usually reaching a level by 10 days that was equal to or slightly lower than that after the first dose. The secondary rise in blood pressure was more frequent and pronounced in patients who had larger initial responses and higher renin levels. The transient increase in pressure could not be explained by failure of blockade of the renin system, since aldosterone excretion was maximally suppressed by the second day of treatment. The data indicate that both the blood pressure response to the first dose and the renin level can be used to predict the long-term response to captopril.