Two experiments with Jersey cows grazing good quality tropical pasture were conducted to study the effects of feeding a commercial concentrate on milk production. The first experiment was carried out in mid-lactation when the cows were fed 0·5kg of concentrate per kilogramme of milk after the first 4·5 or 9·0kg of milk. A response of 0·36kg of milk per additional kilogramme of starch equivalent (SE) was recorded. Experiment 2 was conducted during the first 20 weeks of lactation with feeding rates of 6·8, 4·5 and 2·3, and 5·5, 3·2 and 0·9kg of concentrate per cow per day for the first ten and the second ten weeks of lactation, respectively. The average response was 0·74kg milk per additional kilogramme of SE. Level of concentrate feeding had little or no effect on persistency of milk yield, but the highest level of concentrate in experiment 2 significantly decreased butterfat percentage. The responses recorded to supplementary feeding are in accord with recent data on cow nutrition and pasture management in temperate regions.