Normal and castrated male rats implanted with permanent right atrial cannulae were used, and sequential blood samples were taken every 2 min through the cannula. When the rats, both normal and castrated, were exposed to ether for 2 min, the prolactin (Prl) concentration in plasma was immediately and dramatically elevated. During 40 min of continuous ether anesthesia, plasma Prl concentration was not sustained. After the initial ‘surge’ of Prl secretion, which lasted about 15 min, the concentration returned to normal levels. Following the peak of the Prl ‘surge’, Prl disappeared from the circulation with a calculated half-life of 7 min (mean value: 6.9 min; confidence interval: 6