Six young female fallow deer, including 3 that were ovariectomized at 9 months of age, were blood sampled at frequent intervals after i.v. injections of (1) ACTH analogue (tetracosactrin), (2) GnRH analogue (buserelin) and (3) saline solution on separate occasions at 11, 13, 15 and 18 months of age. Relative to prechallenge plasma values, ACTH administration resulted in a 4-10-fold increase in mean plasma progesterone concentrations, but only a 10-45% increase in mean plasma cortisol concentrations, within 40 min for entire and ovariectomized does during the prepubertal periods (11, 13 and 15 months) and for ovariectomized does during the post-pubertal period (18 months). Post-pubertal entire does exhibited high mean basal plasma progesterone concentrations (3-4 ng/ml) indicating a luteal source of secretion, with the ACTH-induced progesterone response being additive to the luteal progesterone but of similar magnitude to responses in the ovariectomized does. There was no significant ACTH challenge effect on mean plasma LH concentrations for entire or ovariectomized does at all ages. GnRH administration had no significant effects on mean plasma concentrations of progesterone and cortisol of entire and ovariectomized does, although there was a small increase in mean plasma progesterone values in post-pubertal does that may have reflected a luteal response to GnRH (via LH). GnRH challenge resulted in marked increases in mean plasma LH concentrations but the response patterns were different for the 2 types of does, being more rapid and of higher magnitude for ovariectomized does.(ABSTRACT TRUNCATED AT 250 WORDS)