Oral administration of a single 1 g dose of MIT to 10 normal male and female subjects resulted in a rise in serum prolactin in each subject. The mean peak level of serum prolactin attained by the 10 subjects was 36.3 ± 7.9 ng/ml which was highly significantly elevated (P < 0.0005) above the mean basal level of 5.3 ± 1.0 ng/ml. While there was no significant difference between the basal serum prolactin levels of male and female subjects, the mean peak level attained by male subjects following MIT (18.8 ± 3.3 ng/ml) was significantly less (P < 0.0025) than that recorded for the female group (62.5 ± 9.1). Serum levels of growth hormone (GH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH), triiodothyronine (T3), thyroxine (T4) and cortisol were not significantly altered following MIT administration. The complete absence of side effects due to MIT make it a suitable drug for the acute clinical assessment of pituitary prolactin reserve.