Abstract:Treatment of neonatal, but not adult, rats with glucocorticoids decreases the rise in pineal serotoninN‐acetyltransferase activity upon stimulation with β agonists. Pineals in organ culture and exposed to steroids also show a dose‐dependent decrement in response to β agonists which increases with steroid exposure time. Pineals from neonatal and adult animals are equally sensitive. The effects of steroids on pineals in organ culture appear to be reversible, and the order of potency of different steroids differs from that observed when steroids are administered in vivo. Both in vitro and in vivo steroids appear to act at a site after cyclic AMP generation. Hydroxyindole‐O‐methyltransferase activity in the adult pineal does not appear affected by steroid