Circadian rhythm in the plasma levels of prolactin in goldfish,Carassius auratusL

Circulating prolactin hormone levels demonstrate a marked circadian rhythm in tapwater‐ and 30% seawater‐acclimated Carassius auratus L. The levels of prolactin were essentially similar in fish in both ambient media except at 12.00 hr when the levels in tapwater fish were significantly (p<0.01) lower than in tapwater‐ and seawater‐acclimated animals at any other time period. The implications of the prolactin hormone levels are discussed in relation to the involvement of prolactin in freshwater osmo(iono)regulation in this species. There was no significant circadian rhythm in the content of acid phosphatase in the prolactin‐secreting cells.