Using in situ hybridization, we have investigated the presence of mRNA coding for growth hormone (GH) in the rat brain. Using 32P-labeled GH cDNA as a probe, the pituitary gland showed hybridization in unfixed sections. Using 3H-labeled GH cDNA hybridized to fixed sections, only cells in the anterior pituitary were labeled in good agreement with the localization of somatotropes. In the brain, wide zones were labeled with 32P-GH cDNA: the caudate putamen, the striatum, the ventral thalamus, the formatio reticularis and the basal cortex. With the 3H GH-cDNA probe, more discrete regions of the brain showed hybridization: the basal cortex, the outside part of the hippocampus and the caudate putamen.