Abstract This study was designed to evaluate responsiveness and sensitivity of pituitaries from chickens to growth hormone (GH) secretagogues during late embryonic development. Anterior pituitary cells from 16-, 18- and 20-day-old chicken embryos were subjected to reverse hemolytic plaque assays (RHPAs) for GH in the presence of GH-releasing hormone (GHRH) and thyrotropin-releasing hormone (TRH). The proportion of somatotrophs detected increased between embryonic days 16 and 20, from 16 to 19.5% of all pituitary cells. Sensitivity to GHRH and TRH was similar and increased between embryonic days 16 and 20. On embryonic day 16, about 50% of somatotrophs that were not detected under basal conditions released GH in the presence of GHRH at 2 and 6 h. In contrast, only 15 and 30% of day 16 somatotrophs released GH by 2 and 6 h, respectively, following exposure to TRH. Thus, at least one of five somatotrophs responded to GHRH but not to TRH. By embryonic day 20, the proportions of somatotrophs that responded to GHRH and TRH were approximately equal at about 40%. These results provide additional evidence that GHRH and TRH may be involved in hypothalamic regulation of GH secretion during chicken embryonic development. Furthermore, it appears that full differentiation of functional chicken somatotrophs does not occur abruptly but rather gradually between embryonic days 16 and 20, during which a subpopulation of GH cells undergoes changes in sensitivity and responsiveness to TRH.