Plasma hormone concentrations were measured in steers with different potentials for growth and body composition to help determine the physiological bases for these differences. In trial 1, four steers each of large, medium and small type were fed ad libitum a 70% concentrate diet, were housed indoors at 16-18 C and were accustomed to frequent handling. Average weights for the large, medium and small types were 373, 318 and 284 kg, respectively. The average age was approximately 11 months. Blood samples were taken via a jugular catheter every 20 min for a 24-hr period prior to and 30 days after being implanted with a removable estradiol implant. Growth hormone (GH) and insulin were measured in every sample. Glucocorticoids (GC) were measured in samples taken every 2 hr while thyroxine (T(,4)) and triiodothyronine (T(,3)) were assayed in pooled samples;Large type steers had significantly higher (P = .03) overall and baseline mean GH concentrations than either the small or medum type steers. The amplitude of the GH secretory spikes appeared to be greater in the large type steers although differences were not statistically significant. Estradiol implants elevated plasma GH in all steer types (P = .02). Plasma insulin concentrations were higher (P = .01) in small than in either medium or large type steers before implanting. Estradiol elevated insulin concentrations in the large type steers only. Plasma GC concentrations were highest in the small type steers but the differences were not statistically significant. There were no differences in plasma T(,4) but T(,3) was higher (P = .04) in the small type than in the large or medium type steers;A second and similar trial was conducted in which five large and four small type steers without implants were compared. These steers were in a fasted state during the 11-hr sampling period. There were no significant differences in plasma GH, insulin or GCs, although the trends were similar to trial 1 with GH being slightly higher in the large type steers while insulin and GC concentrations were slightly higher in the small type steers. Plasma T(,4) was higher (P = .04) in the large type steers but T(,3) was higher (P = .06) in the small type steers;These results suggest that differences in growth rate and body composition in different types of cattle may be due, at least in part, to differences in plasma concentrations of GH and insulin. No conclusions can be drawn concerning the effects of type or estradiol implants on plasma GC or thyroid hormone concentrations.