Chinese J. Physiol. 31(2):105-112, 1988. The effect of calcitonin (CT) on the release of prolactin (PRL) and thyrotropin (TSH) in vitro from anterior pituitary glands (APs) of thuroidectomized (Tx) and intact male rats was investigated. Male rats were Tx or sham Tx, then sacrificed 47 days later. APs were removed, quartered and incubated with CT (0 or 32 microIU/ml) and/or thyroxine (0 or 0.1 microgram/ml) at 37 degrees C for 4 h. Thyroidectomy significantly decreased PRL release in vitro. CT inhibited the release of PRL in vitro from APs of intact rats but increased PRL release from APs of Tx animals. Neither CT nor T4 affected the release of TSH in vitro. These results suggest that the modulation of CT on the release of PRL from rat anterior pituitary glands is thyroxine-dependent.