SUMMARY: Five monoamine oxidase inhibitors (MAOI) were tested for a correlation between MAO blocking potency, the effect on somatotrophic hormone (STH) production and metabolic effects on the pituitary. Mebanazine, N-acetyl-mebanazine and pargyline inhibited STH production, body weight gain and glucose-6-phosphate (G-6-P) dehydrogenase activity. MAOI which contain a hydrazine group (mebanazine and N-acetyl-mebanazine) also inhibited alkaline phosphatase activity as well as DNA and RNA formation in the pituitary, while pargyline, which contains an amino group, was inactive in these respects. Iproniazid and norpargyline were inactive with regard to STH production, body-weight gain and metabolic activities of the pituitary: alkaline phosphatase activity, G-6-P dehydrogenase activity, and DNA and RNA content. This suggests a relation between the ability of an MAOI to inhibit STH production and inhibition of the phosphogluconate oxidative pathway.