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The effect of insulin was tested on the rate of synthesis and release of growth hormone in cultured rat anterior pituitary cells. Concentrations of insulin between 10^–9 and 10^–7 mol/l (6–600 ng/ml or 0·15–15 mu./ml) inhibited synthesis of growth hormone; 10^–8 mol insulin/1 was most effective. The effect was observed after a time-lag of at least 1 h. Insulin at concentrations between 3 x 10^–9 mol/l and 3 x 10^–7 mol/l also inhibited growth hormone secretion in 30 min incubations. The most effective insulin concentration in this case was 3 x 10^–8 mol/l. Insulin (10^–9–10^–7 mol/l) also decreased the intracellular content of prostaglandins E and F. The effect was rapid, reaching a maximum after 30 min. Indomethacin, an inhibitor of prostaglandin synthetase, dramatically lowered the concentration of prostaglandins in the cells within 30 min; growth hormone synthesis was also decreased, but not until after 2 h of incubation. The results suggest that an initial response to insulin treatment is a lowering of intracellular levels of prostaglandins, which may then mediate a decrease in growth hormone synthesis, after a 1–2 h delay.