We report the results of an observer blind crossover trial in 31 patients with essential hypertension comparing the hypotensive effect of four doses of prazosin and of hydralazine used as the third step of triple therapy and additional data on the efficacy of low dose combinations of these two drugs. Both drugs had an appreciable antihypertensive effect, 1 mg of prazosin being equivalent to 12.5 mg of hydralazine. Increasing the dose of prazosin from 4 to 8 mg twice daily and hydralazine from 50 to 100 mg twice daily did not consistently reduce blood pressure. The combination of the two drugs at low doses resulted in the blood pressure of more patients being controlled (diastolic blood pressure 90 mm Hg or below) than when either drug was used alone at high dose. The symptoms reported by patients were similar for both drugs. The combination did not lead to an increased reporting frequency. The trial demonstrates one of the problems of a crossover study, namely a treatment/period interaction.