The location and quantity of the insulin receptor and its associated tyrosine kinase activity have been examined in the forebrains of lean (Fa/?) and obese (fa/fa) Zucker rats using immunocytochemistry (ICC) and biochemical procedures. These studies were performed in conjunction with ICC and Western blot analysis of phosphotyrosine-containing proteins (PY-proteins). The results from ICC show a similar distribution and content for the insulin receptor among forebrain regions of lean and fatty Zucker rats. Biochemical analysis of the receptor was conducted on the hippocampus. Insulin binding studies using lectin-purified receptor extracts demonstrated similar receptor number and comparable hormone binding affinity for lean and obese animals. Autophosphorylation studies with the receptor extracts from the two groups did not find any differences in the tyrosine kinase activity of insulin receptors. In contrast to the normal findings with the insulin receptor, an abnormality in the obese animals was evident in the content of PY-proteins detected by ICC in the hippocampus, piriform cortex and olfactory bulb. Neurons in these brain regions showed a reduction in staining by an antibody against PY-proteins. Furthermore, Western blots of hippocampal extracts from obese rats demonstrated a reduction in phosphotyrosine content of two proteins of Mr 180 and 130 kD. These findings point to a previously unrecognized alteration in the CNS of the obese, insulin-resistant Zucker rat.