The onset of summer anagen in mink is initiated by increasing photoperiod and is correlated with elevated serum prolactin (PRL) concentrations. Exogenous PRL advances, whereas inhibition of PRL secretion delays and/or blocks summer anagen.  The objectives of this research were to (1): determine serum PRL levels in mink exhibiting natural and artificially induced (adrenalectomy, ADX; long photoperiod, 16L: 8D; and haloperidol, HAL), summer anagen.  In addition, serum PRL levels were determined in mink treated with melatonin (MEL), to inhibit PRL secretion and block summer anagen, both alone and in combination with HAL and 16L: 8D photoperiod, and (2): determine skin PRL receptor (PRL-R) levels in control, ADX and MEL treated animals. There was no difference in time of onset of summer anagen between control and 16L:8D, even though serum PRL levels were elevated 4 weeks earlier than controls in response to the long photoperiod (P<0.05).  Mink treated with HAL or HAL + MEL, displayed an earlier rise in serum PRL levels (P<0.05), but onset of anagen was similar in both groups. Serum PRL levels were non-detectable in MEL and 16L: 8D + MEL treated mink, and none of these animals entered summer anagen between Feb 13 and July 24.  ADX’d mink entered summer anagen approximately 1 month earlier than controls and there was no difference in serum PRL levels between the two groups.  On September 29, after completion of summer anagen,  there was no difference in skin PRL-R concentrations between control, ADX, and MEL-treated mink.  Interestingly however, in mink exhibiting winter anagen in response to MEL, skin PRL-R levels were higher in anagen skin (8.8 + 0.21 fmol/mg) than in telogen skin (3.85 + 1.17 fmol/mg; P<0.01).  Because serum PRL levels were not different between any of the MEL-treated mink, suggests to us, that MEL stimulates production of the PRL-R in mink skin during the anagen phase of the hair growth cycle, only.