In order to compare the effects on reproductive performance of short-term or prolonged exposure to elevated temperatures during vitellogenesis, female Atlantic salmon Salmo salar were held at a water temperature of 22ࠃ for periods of 4 or 6 weeks during the austral summer and autumn. Plasma levels of 17߭oestradiol (E2), testosterone (T) and vitellogenin (Vtg) were monitored and reproductive success was compared to that in groups of fish maintained at 14 or 22ࠃ for 12 weeks from mid-January. Significant endocrine effects were observed within as few as 3 days of the commencement of exposure to 22ࠃ, when plasma levels of E2 (c. 0絠ng ml-1) and Vtg (c. 1素mg ml-1) were approximately half those observed in fish maintained at 14ࠃ (c. 1簠ng ml-1 and 2締mg ml-1 respectively). The fertility and survival to the eyed stage of ova from fish held at 14ࠃ exceeded 85 and 70% respectively, whereas ova from fish held at 22ࠃ for 6 or 12 weeks exhibited significantly reduced fertility (<70 and <45% respectively) and survival (c. 40 and 13% respectively). In spite of significant endocrine effects at all stages, a 4 week exposure to 22ࠃ only generated significant reductions in egg fertility (<65%) and survival (c. 30%) when it occurred between mid-February and mid-March. Together, these data confirm that high temperature spikes can affect reproductive success as strongly as more prolonged exposures, and indicate that there is a critical period of reproductive sensitivity to elevated temperature in late February and early March in this stock of Atlantic salmon.