1 Prolactin levels in response to sulpiride were studied in healthy volunteers. 2 Oral doses of 1 mg-50 mg sulpiride or placebo were given. 3 A 3 mg sulpiride dose produced similar levels to those achieved with both 10 mg and 50 mg. 4 Circadian effects were studied showing no significant differences in the prolactin response to sulpiride. 5 Acute or chronic responses showed an attenuation with chronic sulpiride treatment to 50% of the peak levels attained with acute treatment. 6 These results indicate that sulpiride retains its potent ability to produce prolactin release even at the low doses studied here.