Milk production, plasma bovine somatotropin (bST) and insulin-like growth factor I (IGF-1) were measured in dairy cows following a single subcutaneous injection of a slowly released preparation of either recombinant enterokinase linker bST (somidobove: 640 mg) or recombinant methionyl bST (sometribove: 500 mg). There was a 3-7-fold increase in plasma bST concentrations during the first three postinjection hours in cows treated with both sometribove (from 3.4±0.8 to 11.2±3.0 ng ml-1) or somidobove (from 2.3±0.3 to 17.5±2.6 ng ml-1). In the next 8 days the bST concentration in the bST-treated cows varied, but was still significantly increased above the controls. In the following days, the concentrations of bST did not differ from the controls. Plasma concentrations of IGF-1 increased nearly 2-fold as early as 24 h following recombinant bST administration and then continued to rise so that by 48 h postinjection they were nearly four times higher (control 16.2, bST-treated 61.7 ng ml-1). From 48 h after sometribove injection, IGF-1 concentrations remained at a plateau (varying between 60.4 and 85.7 ng ml-1) till day 11. Then it decreased slowly, but still remained higher on day 14 than those in placebo-treated cows (44.4±17.8 ng ml-1 in bST-treated animals; 12.2±7.5 ng ml-1 in the controls). Although IGF-1 level was increasing in all bST-treated animals, the absolute IGF-1 increase was not related to the increase in milk production.