Double labelling immunohistochemistry using antibodies to thyrotropin releasing hormone (TRH) and somatostatin (SS) was undertaken in the anterior hypothalamus in 6 rats. Light microscopic quantitation revealed that 94.5% of SS immunopositive perikarya in the preoptic anterior hypothalamic area ( PO AHA) and 97.5% in the paraventricular nucleus appeared to be contacted by one or more TRH immunopositive terminals. In the chronically cannulated unanaesthetised male rat, unilateral microinjections of a range of doses of TRH were made in the PO AHA, where SS neurons are located, or in the medial basal hypothalamus, where growth hormone (GH)-releasing factor (GRF) neurons are located. Transient reductions in GH plasma levels occurred only after injections of the highest (10 nmol) dose of TRH in both sites. The function of TRH inputs to both somatostatin and GRF neurons appears to be inhibitory for GH. The physiological conditions in which these inputs function remain to be defined.