The antisecretory effects of a new histamine H2-receptor antagonist, ranitidine hydrochloride, have been investigated on basal and pentagastrin-stimulated acid secretion in healthy volunteers 5 and 10 h after oral administration of 150 mg. In addition, the 24-h intragastric pH-profiles have been measured in patients undergoing parenteral nutrition after three doses of 150 mg ranitidine per day. A 40% inhibition of basal acid output has been noted even 10 h after drug intake. The intragastric pH-values were raised above 5 for at least 24 h. The new H2-antagonist ranitidine has been proven to be a potent and long-acting antisecretory compound.