In a subtropical climate, high milk-yielding dairy cows were kept during the summer under ventilated conditions or untreated; during the winter, cows were kept untreated. The afternoon mean rectal temperature for multiparous cows in the three groups was 39.3, 39.8 and 38.9 degrees C, respectively. Each group was significantly different from the other two (P less than 0.05). Plasma LH concentrations measured every 6 h during the oestrous period in 38 'summer' cows were not significantly different for untreated and ventilated animals. Conception rate was higher (P less than 0.05) in cows that showed oestrous behaviour before the LH surge reached its peak than in cows in which oestrus coincided with or occurred later than the LH surge. Plasma progesterone levels measured in 62 cows during the oestrous cycle before the first insemination were higher in the winter than in the summer in multiparous, but not in primiparous, cows. Ventilation increased progesterone levels in multiparous and primiparous cows. Plasma oestradiol-17 beta levels did not differ between groups until 36 h before the onset of oestrus, when they remained at 4.75 pg/ml in winter and summer-ventilated cows but increased to 6.75 pg/ml in summer untreated cows (P less than 0.01). Significant negative correlations were found between oestradiol levels observed 12 h before to 12 h after the onset of oestrus and plasma progesterone concentration during both the preceding and the subsequent oestrous cycles.