Peripheral plasma concentrations of LH, oestradiol-17\g=b\ and progesterone were measured in 13 mature swamp buffalo cows at 4-h intervals from 36\p=n-\40h before until 36\p=n-\40h after the onset of oestrus. Mean LH concentrations increased sharply to a peak of 35 ng/ml and returned to basal levels of 5 ng/ml within a 12-h period beginning soon after the onset of oestrus. Mean oestradiol-17\g=b\ concentrations were within the range 9\p=n-\13pg/ml from 36\p=n-\40h before until 12\p=n-\16h after the onset of oestrus, and within the range 7\p=n-\9 pg/ml thereafter. Progesterone concentrations remained around 0\m=.\1ng/ml throughout the sampling period. There were no significant differences in hormone concentrations or changes between cows that conceived and those that did not conceive to artificial insemination 12\p=n-\24h after onset of oestrus.