Adult male and female Fundulus heteroclitus (killifish) between 7 and 9 g body weight were included in this work, which was designed to determine the effects of hypophysectomy upon pectoral fin regeneration.Consideration was given to a possible relationship between the pituitary gland and fin regeneration and whether or not the presence of the pituitary is essential for regeneration. Included in this investigation were 436 killifish which consisted of three hypophysectomy, two sham hypophysectomy, and one control regenerate series. In the experiments, hypophysectomy was (a) concomitant with fin amputation; (b) followed in 5 days by fin amputation; or (c) performed 5 days after fin amputation.The results show that after hypophysectomy long-term regeneration ensues and the resulting morphogenetic pattern is very similar to that found in regenerates of the sham hypophysectomized and control regenerate killifish.