Hormonal effects of prolonged administration of gonadotrophin-releasing hormone (GnRH) were investigated in 7 fertile stallions in winter and summer. The stallions were divided into 4 groups so that 1 animal received 0.625 micrograms of GnRH and each of 2 animals received 1.25, 2.5 or 5.0 micrograms of GnRH subcutaneously every 30 min for 5 days. Daily blood samples were collected from 5 days before to 5 days after treatment for measurement of plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH), testosterone (T) and oestrogen conjugates (EC). Five-minute blood samples were collected over a 4 h period prior to, and on the last day of, GnRH treatment in each season for analysis of pulsatile changes in plasma LH concentrations. Mean concentrations of LH and T prior to treatment were higher (P less than 0.05) in summer than winter, whereas plasma FSH and EC concentrations did not differ between seasons. Increasing the dose of GnRH appeared to increase plasma concentrations of LH, FSH and T above baseline in winter (P less than 0.05), whereas a significant effect was not observed in summer. Plasma EC concentrations were not affected by treatment in either season. In GnRH-treated stallions, plasma LH and T concentrations were positively correlated such that the profiles of these hormone concentrations appeared to change in a similar direction over time in winter (P less than 0.01) but not in summer. Plasma concentrations of FSH and EC were not correlated in either season. A positive correlation between plasma LH and FSH concentrations was detected in both winter (P less than 0.01) and summer (P less than 0.02). In contrast, changes in plasma LH and T and plasma FSH and EC concentrations were not correlated over time in either season in control stallions. A positive correlation between plasma LH and FSH concentrations in control stallions was observed in summer (P less than 0.03) but not in winter. Prior to treatment, pulse patterns of LH were similar in both seasons. Following administration of GnRH, irrespective of dose, pulse frequency increased (P less than 0.05) and pulse duration decreased (P less than 0.05) resulting in a decrease in the total amount of LH released per pulse (P less than 0.05) in winter only. Amplitude and interval were not affected by GnRH treatment in either season. The results of this study suggest that the pituitary gland and testes of fertile stallions are more responsive to administration of prolonged pulsatile GnRH in winter than in summer.