A double-blind, randomized trial of oral vs intravenous clomipramine hydrochloride pulse-loading dosing regimens was conducted. After a two-week drug-free assessment period, 22 inpatients with a diagnosis of major depressive disorder were given either an evening infusion of 150 mg of clomipramine hydrochloride and placebo tablets or 150 mg of oral clomipramine hydrochloride and an isotonic saline infusion. Twenty-four hours later, this procedure was repeated using a dose of 200 mg of clomipramine hydrochloride. Patients received no further medication over the next five days. The mean Hamilton Depression Rating Scale score for all patients, five days after pulse loading, had dropped by 35% (range, 13.3% to -82.4%). This improvement was significant, as was the amelioration in the Raskin Severity for Depression Scale and the Beck Depression Inventory scores. Although the bioavailability of parenteral clomipramine was greater, there were no significant differences in either efficacy or side effects between the two groups. Pronounced early improvements in severe depressive symptoms may be achieved via loading dose regimens with clomipramine in the absence of continuous treatment.