Corticosterone, Prolactin, and Growth Hormone Responses to Handling and New Environment in the Rat*

&NA; Temporal characteristics of hormonal responses to handling and novel environment were studied in the rat in order to determine whether the same factors which modify corticosterone responses, i.e., time of day and type of stimulus, also modify prolactin and GH responses. Resting corticosterone showed the expected difference between crest and trough of the diurnal cycle. In contrast, GH and prolactin showed no difference at these times. Prolactin elevation occurred in response to the same stimuli which produced adrenal activation. Prolactin responses, however, differed from adrenal responses in being more rapid at the trough of the adrenal cycle and slower at the crest. In contrast to prolactin and corticosterone, GH showed a drop (nonsignificant) following stimulation. It is concluded that prolactin and corticosterone both respond to identical stimuli but that the prolactin response shows different characteristics from the adrenal response suggesting that different regulatory mechanisms are involved.