Suppressors of cytokine signalling (SOCS) negatively regulate cytokine-induced signalling pathways and may be involved in leptin and prolactin (PRL) interactions. Herein, we examined the effect of PRL on SOCS-3 mRNA expression in pituitary explants and investigated whether leptin could modify the expression of SOCS-3 mRNA in pituitary explants. In the first experiment, we used pituitaries isolated from 16 ewes decapitated in March, May, July and October (four per month). Tissues were cut into 50 mg explants, which were treated with control or medium containing PRL (100 or 300 ng/ml). Incubation was maintained for different time intervals: 0, 60, 120, 180, 240 or 300 min. Real-time PCR was used to measure SOCS-3 mRNA levels. In the second study, we used 24 ewes surgically fitted with third ventricle cannulas (12 were used during the long-day period, and 12 were used during the short-day (SD) period). Each ewe was administered an i.c.v. injection of Ringer-Locke buffer or leptin (0.5 or 1.0 μg/kg body weight). Explants of anterior pituitaries were collected and snap frozen 1 h after injection. Semi-quantitative expression of SOCS-3 mRNA was performed using reverse transcription-PCR. PRL stimulated SOCS-3 expression in the pituitaries collected in March (P<0.05) and May (P<0.01 and P<0.05 for lower and higher doses respectively), inhibited SOCS-3 expression in pituitaries collected in July (P<0.01) and had no effect in pituitaries collected in October. Treatment with leptin increased SOCS-3 expression during the SDs in a dose-dependent manner (P<0.01). The results demonstrated that photoperiod may be involved in leptin and PRL effects on SOCS-3 expression in sheep.