Serum growth hormone (GH) levels and growth rates of goldfish were measured concurrently following discrete electrothermic lesions of a number of hypothalamic nuclei. Following lesions of the nucleus preopticus periventricularis (NPP), significant increases in serum GH levels were evident at 4 weeks postlesioning in comparison to both sham-operated and normal control groups. In addition, the NPP-lesioned fish showed significant increases in both total body weight and standard length increments compared to control groups. Lesions in other hypothalamic areas including the nucleus anterior tuberis, nucleus lateralis tuberis and the nucleus recessus lateralis had no consistent effect on growth rates or serum GH levels. The present results suggest that the NPP is concerned with the inhibition of GH secretory function in the goldfish.