Abstract: Growth hormone receptor (GHR) signaling pathway plays an important role in postnatal growth of animals. Although significant progress has been made in elucidating the signaling pathways activated by GHR in recent years, a comparative expression analysis of all the GHR signaling related genes and evaluation of GH-signal activation (GHSA) in an in vivo model still remain elusive. The zebrafish (Danio rerio) is an excellent model organism to study both developmental and physiological processes. In the present study, we comparatively analyzed the expression of GHR signal related genes, such as gh, ghra, ghrb, jak2a, jak2b, stat5.1, stat5.2, igf1, c-fos, socs1 and socs2 in adult tissues, and during embryonic development by means of RT-PCR. Maternal expression was found in most of them and their zygotic expression mostly started from early-somite stage, far before the differentiation of pituitary somatotrophs and before the establishment of a functional circulatory system. In adults, the tissue distribution abundance of socs mRNA was negatively correlated with the GH-signal targets, igf1 and c-fos. By analyzing the transcription level of c-fos and igf1 and the promoter activity of spi2.1, in zebrafish embryos injected with different constructs, i.e., the expression constructs of gh and ghr, we established an in vivo model to evaluate GH-signal activation (GHSA) in early development of zebrafish embryos. Overexpression of gh or ghr alone in zebrafish embryos could significantly stimulate GHSA at 1 day post-fertilization (dpf) and 3 dpf, strongly indicating the existence of functional GH and GHR proteins in zebrafish embryos from 1 dpf, far prior to the formation of functional pituitary gland. A synergetic overexpression of gh and ghr could amplify the GHSA effects of gh or ghr overexpression.