An ultrastructural study of the adenohypophysis, after exposure of female Wistar rats to a signalled unpredictable 5-and I5-day stress regimen, is described. Cellular activity of the adenohypophysis correlated well with the circulating levels of corticosterone. Intense secretory activity was observed in all tropic cell types at 5 and 15 days although the observed differences generally were greater in the 5-day stressed group. It was observed that the oestrous cycles of 40 and 100 % of the rats became desynchronized over the 5-and I5-day stress period respectively.