The antisecretory effects of a new histamine H2-receptor antagonist ranitidine hydrochloride 150 mg, have been tested on basal and pentagastrin-stimulated acid secretion in healthy volunteers 5 and 10 h after oral administration. In addition, the 24 h intragastric pH-profiles have been measured in patients undergoing parenteral nutrition after three oral doses of ranitidine 150 mg per day. A significant inhibition of basal acid output has been observed up to 10 h after the drug. The intragastric pH remained above 5 for at least 24 h. Ranitidine has proven to be a more potent and longer acting antisecretory compound than cimetidine.