The aim of the study was to determine the effect of exogenous melatonin treatment on circulating prolactin levels in red deer. Melatonin was administered from 12 June 1984 (day 1) to lactating and non‐lactating hinds in the feed daily at 1600 h, and to non‐lactating hinds by a subcutaneous implant. Average concentrations (ng/ml) of prolactin in plasma taken serially over 15‐h periods were significantly higher for untreated hinds than for melatonin‐treated animals on day 15 whether lactating (66–133 v. 23–28,P<0.05) or non‐lactating (28–174 v. 8–13,P<0.01), remained higher on day 36 (lactating: 41–152 v. 15–21,P<0.05; non‐lactating: 21–50 v. 1–7,P<0.001) but had decreased to similar levels on day 72 (lactating: 5–24 v. 7–17; non‐lactating: 2–9 v. 0–4).The advanced reduction in plasma prolactin for all melatonin‐treated hinds was associated with an advan