Lactating goats routinely milked twice daily were used. Starting at peak lactation, two groups of five goats were milked three times daily unilaterally (right gland) for 6 weeks, then one group received recombinant bovine somatotrophin (BST) by daily injection for 6 weeks, and finally both groups were returned to unilateral thrice-daily milking (left gland) for 6 weeks. Thrice-daily milking increased milk yield significantly by 10·4%. As thrice-daily milking finished and BST treatment began there was a small (2·7%) but significant increase in whole-udder milk yield in the treated goats and a decline in the control goats; throughout the BST treatment period treated goats produced 14·3% more milk than controls. The effect of BST was the same in glands previously milked thrice daily as in those previously milked twice daily. Milk yield declined as BST treatment finished, but this decline was less in the glands that then began thrice-daily milking. The effect of milking frequency was similar in this period to that obtained earlier in the experiment, and was not different between goats that had previously received BST and those that had not. The results are discussed in relation to the proposed modes of action of BST and frequent milking, and possible interactions between the two.