Effects of thyrotropin-releasing hormone (TRH) on growth hormone (GH) secretion were investigated in vivo (on intact or mediobasal hypothalamic lesioned rats tested under either anesthesia or free moving conditions) as well as in vitro (in incubation or perifusion systems of anterior pituitary tissue). The peptide induced a rapid, dose-dependent increase of plasma GH levels in free moving animals bearing an extensive lesion of the mediobasal hypothalamus including the median eminence. Under comparable conditions, TRH was ineffective in intact animals. After chloral hydrate anesthesia a GH response to TRH was recorded in both groups, but lesioned rats exhibited a better responsiveness to all doses tested. In vitro TRH increased GH release from incubated or perifused pituitaries sampled from both intact and lesioned rats in a transient and concentration-dependent manner. A similar effect was obtained with the (3 Me His<sup>2</sup>) analogue of TRH. These findings indicate that TRH can affect GH secretion at the pituitary level under specific experimental conditions and support the hypothesis that either peripheral hormones or other, still unidentified hypothalamic neurohormones may modulate this effect.