The effect of administration of bovine somatotrophin (bST) on peripheral conversion of thyroxine (T4) to tri-iodothyronine (T3) was studied in non-pregnant lactating Holstein cows. Six cows were injected daily for 5 days with 40 mg recombinantly derived bST, while six control cows received excipient alone. Blood samples were collected hourly from 08.00 to 19.00 h on a single day the week before treatment and on days 4–5 of treatment. All other tissue samples were obtained at slaughter, 20–23 h after the last injection. Administration of bST increased milk production and caused a 9% increase in hepatic DNA. Consumption of feed did not differ between control and bST-treated cows. Treatment did not alter serum concentrations of T4 or T3, although concentrations of thyroid hormones in the serum increased from 08.00 to 19.00 h. Activity of thyroxine-5′-monodeiodinase (5′-D) in liver and kidney was similarly unaffected. However, activity of 5′-D in mammary tissue increased approximately twofold in response to bST administration. We suggest that an increase in mammary conversion of T4 to the more biologically potent thyroid hormone T3 plays a role in mediating the galactopoietic response of dairy cattle to bST. Journal of Endocrinology (1989) 121, 205–211