Plasma somatotropin (GH) and somatomedin C (IGF,) concentrations were measured by radioimmunoassay in 3-day old and 10-day old calves intravenously injected with growth hormone releasing factor (GRF) 1-44, GRF 1-29 or thyrotropin-releasing hormone (TRH). In 3 day old animals the increase in plasma GH concentration was GRF 1-44 doserelated (50, 100, 200 pmoles.kg-' of body wt). In four 10-day old calves injected with the lowest dose, the increase in plasma GH concentration was not different from

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... that observed in four 3-day old animals treated with 200 pmoles.kg-1 of body weight. However, the response observed in four 3-day old calves injected with GRF 1-29 (50 pmoles.kg-1 of body wt) was not different from that observed following the same treatment in four 10-day old calves. In four 3-day old calves TRH (10 nmoles.kg-1 of body wt) induced a significant rise in plasma GH, prolactin (Prl), thyroxine (T 4 ) and triiodothyronine (T 3 ) concentrations. The same dose of TRH injected into four 10-day old calves elicited a similar rise in plasma T 3 and T 4 concentrations, but plasma GH and Prl increased less than in 3-day old animals. In three 3-day old or 10-day old calves born spontaneously before term (258-260 days of gestation), the increase in plasma Prl and GH concentrations following TRH was not different from that observed in mature calves of the same postnatal age. Neither GRF 1-44, GRF 1-29 nor TRH elicited any significant change in plasma IGF" insulin or glucose concentration in any group of calves. Introduction.