Synthetic LHRH, at a dose of 5 mug/kg, induced a marked increase in the serum LH level but a marked decrease in the serum prolactin level when injected iv to normal adult female rats on the day of estrus. In adrogenized adult rats which had been showing persistent estrous vaginal smears, a less marked increase in the serum LH level was shown following injection of LHRH. Prolactin level decreased only slightly in these animals. Control level of serum prolactin in normal estrous and androgenized rats were 4.95 +/- 0.35 and 5.42 +/- 0.65 mIU/ml, respectively. Thyroidectomized-normal cycling rats responded to LHRH with a slight decrease in the serum prolactin level when injected on the day of diestrus. Prolactin level increased, however, 5 min after LHRH injection in thyroidectomized-androgenized adult rats, which were thyroidectomized at the age 4, 6 or 10-12 weeks and have retained a low prolactin level similar to that in the diestrous stage of normal cycling rats thereafter. The present results suggest that regulatory system of prolactin secretion became labile by androgenization and this status led to the release of prolactin by LHRH treatment after thyroidectomy.