In a herd of Lucerna cattle, a new dual purpose breed in Colombia it was found in two samples of data covering different periods of time that the weight of milk produced in lactations following birth of a male calf was 3·1 and 4·3 per cent greater than that produced following birth of a female. Of these increases, the amount of milk produced per day contributed l·6 and 3·4 per cent and increased lactation length accounted for l·5 and 0·9 per cent. Results from adjusted least squares means covering all years and accounting for age of cow, season and year effects confirmed these findings. Lactations initiated by abortions produced 21 and 17 per cent less milk than those initiated by normal births. Milk production of in-calf cows was not significantly affected by the sex of foetus, but lactation length was significantly longer when the foetus was female. There was a tendency for calves of the same sex to be born in successive calvings. More female calves were born following a female calf than following a male calf in the ratio of 51·2:48·8. The sex ratio for all births was 51·6 per cent males to 48·4 per cent females. The study covered 11012 calvings.