The daily mean of serum progesterone in cyclic ewes (N = 5) as well as the profile characteristics of progesterone and cortisol in response to an acute single dose (5 i.u./kg liveweight 0.75) of adrenocorticotrophic hormone (ACTH) into ovariectomized ewes (N = 4) was investigated during exposure to a constant thermoneutral temperature of 18 +/- 1 degree C or to a daily cyclic heat stress temperature of 18 degrees C-35 degrees C-18 degrees C, in an environmental chamber. Serum collected daily from the cyclic ewes was assayed for progesterone, while serum collected more frequently for 10 h, on the 14th day of exposure to the respective temperature, from the ovariectomized ewes was assayed for progesterone and cortisol by RIAs. In cyclic ewes, heat stress increased the area under the daily progesterone curve (P less than 0.09) but had no effect on progesterone concentration after the regression of the CL. In ovariectomized ewes, ACTH significantly elevated the response of both cortisol and progesterone (r = 0.75, P less than 0.001) within 10-15 min of injection. In the ovariectomized ewes and during heat stress, the responses of progesterone and cortisol to ACTH were characterized by an initial acute rise, a transient drop, a steep elevation and a gradual but prolonged decline. During thermoneutral temperatures, this biphasic response pattern was not observed.(ABSTRACT TRUNCATED AT 250 WORDS)