Plasma GH and insulin concentrations following feeding as well as following stimulation with GH-releasing agonists were compared between milk-fed (3-wk old) and concentrate + hay-fed (12-wk old) calves. Feeding of a milk replacer diet, but not the concentrate + hay, significantly increased plasma GH, insulin and glucose concentrations. When calves were maintained on a milk replacer diet until 12 wk of age, postprandial plasma GH concentration was not different from that in the age-matched weaned group. It is concluded that postprandial levels in plasma GH and insulin concentrations are altered after weaning.