This study tested the hypotheses that: (1) either acute stress induced by Burdizzo castration, or cortisol infusion would modulate plasma glucose, insulin and growth hormone (GH) concentrations; and (2) immune modulation induced by cortisol would be dependent on the pattern, intensity and duration of circulating cortisol concentrations. Fifty 9.2-month-old Holstein x Friesian bulls (232 +/- 2.0 kg) were blocked by weight and randomly assigned to one of five treatments (n = 10 per treatment): (1) sham handled control; (2) Burdizzo castration; (3) hydrocortisone infusion to mimic the castration-induced secretion pattern of cortisol; (4) hourly pulse infusion of hydrocortisone; and (5) sustained infusion of hydrocortisone for 8h. Blood samples were collected intensively on day 0, and weekly from days 1 to 35. Castration acutely increased plasma cortisol, GH and haptoglobin concentrations, suppressed lymphocyte in vitro interferon-gamma (IFN-gamma) production, but had no effect on plasma glucose and insulin concentrations. Cortisol infusion to simulate the castration-induced secretion pattern of cortisol, and pulse infusion of cortisol did not suppress the IFN-gamma production. A sustained infusion of cortisol resulted in the transient suppression of IFN-gamma production. Moreover, the sustained cortisol infusion resulted in increased plasma glucose, insulin and GH concentrations. The overall 14-day feed intakes and 35-day growth rates were not affected by treatments. In conclusion, cortisol infusion to induce immune suppression in vivo occurred only at pharmacological doses. Within physiological ranges, cortisol was not associated with the suppression of immune function, indicating that during castration cortisol per se is not responsible for the suppression of in vitro IFN-gamma production.