Pituitary-derived bovine growth hormone (bGH) was administered to Jersey cows during late lactation for 7 d. Milk yield increased significantly during treatment and by a maximum of 49.6% on d 7. The magnitude of the increase was similar to that of mammary plasma flow (47.8 +/- 18.3%) over the same period. By 15-21 d after treatment, both variables had returned to pretreatment values. With respect to milk composition, bGH had negligible effect on lactose and fat concentrations but there were significant decreases in protein, sodium and chloride. Arterial plasma concentrations of bGH increased substantially during treatment, but the associated rise in insulin was not statistically significant. Haematocrit decreased significantly, the lowest value being recorded 3 d after bGH treatment ceased. Mammary respiratory quotient fell progressively after the start of bGH treatment and reached the lowest recorded value 3 d after treatment ceased (62.2 +/- 7.3% of pretreatment value). Glucose and acetate uptake by the mammary gland increased significantly during treatment, increase in glucose uptake being due both to a greater arterio-venous difference and to mammary plasma flow. There was strong evidence that the acute response in increased milk yield was associated with multiple effects in terms of mammary plasma flow and metabolism, as well as haematocrit changes indicative of increased plasma volume.