Plasma corticosterone and prolactin concentrations were measured in cycling rats during diestrus, lactating rats continuously with their litters, and 12 h litter-deprived lactating rats under conditions in which both hormones are released. The plasma corticosterone response to ether was twice as high in cycling rats as in lactating rats. Ether was a more effective stimulator of corticosterone release than was suckling and suckling elicited far greater increases in plasma prolactin than did ether. Only a brief suckling stimulus (