The effects of substance P on anterior pituitary secretion were studied in 3 female rhesus monkeys. In nine experiments, 100 μg substance P was injected intraventricularly, and the results were compared to those obtained following intraventricular injection of the control vehicle. In 7 out of 9 experiments, substance P induced a significant increase in prolactin secretion within 5 min. Peak levels at 10 min were approximately 15-20 times those of the baseline control. Substance P also induced a slight but significant decrease in GH secretion 20 min following injection, but at other times GH levels were not significantly changed. LH and FSH as well as cortisol concentrations remained unaltered. In 2 monkeys a decrease in systolic pressure of 40–70 mm Hg within 10-60 sec and lasting 180-300 sec was observed following the administration of substance P but not the control vehicle. The results indicate that substance P, which in the monkey has been shown to be associated with hypothalamic regions implicated in the control of anterior pituitary secretion, can alter prolactin and GH release.