SmithKline Beecham Phase I clinical studies have shown BRL 49653C to be safe and well tolerated. The most frequently observed adverse experience was headache. Serious adverse experiences of transient asymptomatic accelerated unifocal ventricular rhythm of less than 10 seconds duration were observed in two volunteers. The arrhythmias were noted at 27 minutes and 2 hours and 20 minutes after administration of a single dose of BRL 49653C 2mg. Hotter monitoring was performed in two studies in order to evaluate the arrythmogenicity of BRL 49653C. Study 002, was a placebo controlled, 10 day repeat dose, two-period, period balanced, cross-over study, within each of three dose groups (1, 2 or 5 mg) of BRL 49653C. Study 016 was a single dose, double-blind, randomized (with respect to order of placebo), placebo controlled, five period cross-over, oral dose rising study in which each subject received a randomly allocated placebo and four doses of BRL 49653C (5, 10, 15, 20 mg). In both of these studies, there was no evidence of protocol- defined incidence of ventricular tachycardia, ventricular premature beats, or supraventricular tachycardia.