The aim of the present study was to test whether oestrous cycle is associated with the hypothalamus-pituitary-adrenal (HPA) axis function. Thus, corticosterone secretion in rats was investigated following lipopolysaccharide (LPS), acute cold-swimming or ether stress or synthetic corticotrophin-releasing factor (CRF) administration throughout the oestrous cycle. Moreover, plasma corticosterone response to cold-swimming stress or LPS administration also was studied at different times of day on pro-oestrus of di-oestrus-I. The following observations were obtained: the morning plasma corticosterone levels in control rats did not differ with the stage of the oestrous cycle; plasma corticosterone levels increased significantly following LPS administration (2 mg/kg, ip) or following acute exposure to cold (4 degrees C)-swimming or ether stress. However, this increase in plasma corticosterone levels was not related to the stage of the oestrous cycle; synthetic CRF injection induced an increase in plasma corticosterone levels constant on di-oestrus-I and pro-oestrus; plasma corticosterone response to LPS administration or acute cold-swimming stress showed diurnal changes, with the lowest values at 18.00 h, which was independent of the oestrous cycle. By showing the unchanged corticosterone response to LPS, to acute stress and to exogenous CRF throughout the oestrous cycle, and the independence of the diurnal pattern of stress response on the oestrous cycle, the present study suggests that the oestrous cycle has no influence on the HPA activity under the present experimental conditions in rats.