Teleosts have two paralogous growth-hormone receptors (GHRs). In vitro studies demonstrated that both receptors bind to and transmit the signal of the growth hormone (GH). However, one of the GHRs (GHR1) was shown to bind more strongly to somatolactin-α (SLα), a fish-specific peptide hormone that is closely related to GH, and is, therefore, termed somatolactin receptor (SLR). In this study, we questioned whether the dual binding of GHR1/SLR causes a crosstalk (reciprocal activation or inhibition) between GH and SLα signals in vivo. For this purpose, we newly established a transgenic medaka that overexpresses GH (Actb-GH:GFP) and assessed its phenotype. The body weight of these transgenic medaka is about twice that of wild-type fish, showing that functional GH was successfully overexpressed in Actb-GH:GFP fish. The transgenic medaka, especially female fish, showed severe infertility, which was a common side effect in GH transgenesis. The skin color, which reflects the effects of SLα most conspicuously in medaka, was similar to that of neither the SLα-overexpressing nor the SLα-deficient medaka, indicating that GH overexpression does not enhance or suppress the SLα signal. We also verified that a transgenic medaka that overexpressed SLα grew and reproduced normally. Therefore, regardless of the in vitro binding relationships, the GH and SLα signals seem not to crosstalk significantly in vivo even when these hormones are overexpressed.