In this study, a heterologous radioimmunoassay (RIA) for grass carp GH has been validated and used to monitor the kinetics of GH release from perifused grass carp pituitary cells. To establish the anatomical specificity of GH antiserum used in this RIA, immunohistochemical staining was performed in grass carp pituitary sections. Somatotrophs recognized by this GH antiserum were located mainly in the proximal pars distalis without overlapping with gonadotrophs located in the same area or with lactotrophs located in the rostral pars distalis. The immunoreactivity of somatotrophs was abolished by preabsorbing GH antiserum with purified grass carp GH, suggesting that the possibility of a cross-reactivity of antiserum with other grass carp pituitary hormones is unlikely. Using 125 I-labeled carp GH as the RIA tracer, parallelism was observed among the displacement curves of grass carp GH standard, grass carp serum, and culture medium conditioned by grass carp pituitary cells, suggesting that this RIA can be used to quantitate grass carp GH levels in biological samples. Using anin vitrocolumn perifusion system, a superactive gonadotropin-releasing hormone (GnRH) analog (d-Arg 6 , Pro 9 -NEt)-sGnRH(sGnRHa, 0.3–30 nM), dopamine (DA, 0.1–10 μM), and the nonselective DA agonist apomorphine (0.1–10 μM) stimulated GH release from grass carp pituitary cells in a dose-dependent manner. The GH-releasing effect of DA was mimicked by the D 1 agonists SKF38393 (0.1–10 μM) and SKF77434 (0.1–10 μM), but not by the D 2 agonist LY171555 (3 μM). In addition, the GH response to DA (1 μM) was blocked by the D 1 antagonist SCH23390 (5 μM) but not by the D 2 antagonist (±) sulpiride (5 μM), suggesting that the GH-releasing action of DA is mediated through receptors resembling mammalian D 1 receptors. Somatostatin-14 (SRIF 14 , 0.01–100 nM), unlike sGnRHa and DA, induced a dose-dependent suppression on basal GH release. At a high dose (100 nM), SRIF 14 also abolished the GH responses to sGnRHa (100 nM), DA (10 μM), and the D 1 agonist SKF38393 (3 μM). These results, as a whole, provide evidence that GH release in the grass carp is under the direct regulation of GnRH, DA, and SRIF at the pituitary cell level. The present study also suggests that DA D 1 receptors are present in grass carp pituitary cells mediating the GH-releasing action of DA.