The effect of chronic administration of bovine growth hormone (GH) on milk production, food intake and live-weight change was evaluated in five sets of monozygotic twin cows on pasture. Purified bovine pituitary GH (specific activity, 0·78 IU/mg) was administered by daily subcutaneous injections (39 IU/day) for a period of 22 weeks (weeks 5 to 26 of lactation). GH treatment resulted in significantly higher yields of milk (23·3 kg/day), fat (0·97 kg/day), protein (0·74 kg/day) and lactose (115 kg/day) compared with the control group (19·8 kg/day, 0·79 kg/day, 0·63 kg/day, 0·99 kg/day). Milk composition did not differ between treatment groups. There was no difference in the intake of cut grass in week 8 but the voluntary intake of the GH group had increased by week 22 (controls, 15·4 kg dry matter (DM) per day and GH group, 17·5 kg DM per day). There were no differences in the rate of live-weight change for the two groups. Serum somatomedin concentrations were significantly elevated in the GH group on weeks 20 to 22 of treatment (0·043 v. 0·135 U/ml). This experiment indicates that cows chronically treated with GH were able to adjust their food intake upwards to sustain a substantial increase in milk production on a diet composed solely of grass.