The objective of the study was to investigate the influence of the growth stimulators Ralgro zeranol and Rumensin on changes in concentrations of certain hormones in the blood of beef bulls. In the first experiment 21 bulls were investigated which were divided into three groups. The first group was control, the second was administered the Ralgro stimulator (3X) and the third the Ralgro and Rumensin stimulators. In the second experiment there were 12 bulls. The animals in the test group were implanted Ralgro five times during the fattening period. Alternate changes, mostly insignificant, were recorded between the groups in the concentration of triiodothyronine and cyclic adenosine phosphate. Thyroxine concentrations were reduced during all investigations of the first and second experiments in the animals stimulated by the Ralgro preparation. A significant increase was recorded in the bulls at the age of 14.3 months in the second experiment (62.8 nmol.l-1, in comparison with 56.9 nmol.l-1). In both experiments an identical trend of insulin content was observed--in seven out of the total number of eight observations the animals treated with the Ralgro preparation had statistically insignificantly higher values. The largest differences were recorded at the age of 11.1 and 14.3 months in the second experiment (3.9 microIU.ml-1 and 3.6 microIU.ml-1). Significant differences were observed in testosterone concentrations in the second bulls of the test group were lower by 1.9+ nmol.l-1 and 3.4++ nmol.l-1, respectively.