The microanatomical location of IGF-1 binding in the rat brain was determined by in vitro autoradiography with slide-mounted sections of frozen brain. Sections incubated in 0.1 nM [¹²⁵I]-iodo-IGF-l produced a dense grain concentration in regions of the autoradiographic image corresponding to the external palisade zone of the median eminence; other hypothalamic regions were not so heavily labeled. This reaction was significantly reduced in the presence of 100 nM IGF-1. Measurement of binding by computer digital image analysis of autoradiographic images showed that specific binding for 1GF-I in the median eminence was 41.3 t 8 × 10 fmol/mm (mean ‡ SEM); nonspecific binding was I 1.9 ± 1.8 × 10 fmol/mm. In contrast, specific binding to other hypothalamic regions was uniformly lower. In a separate experiment, 1000 nM unlabeled insulin was added. Without insulin, specific binding was 23 ± 0.9 × 10 fmol/mm ; nonspecific binding was 8 ± 0.5 × 10 fmol/mm. In the presence of 1000 nM unlabeled insulin, specific binding for [¹²⁵I]-iodo-IGF-l was 23 t I × 10 fmol/mm. The results suggest that a high concentration of receptors for an IGF-1 -like molecule is present in the median eminence.