In the present work we characterized the kinetic properties of [125I]somatostatin pericellular binding sites in the arcuate nucleus of the hypothalamus of the rat by quantitative high-resolution light microscopic radioautography. In order to determine whether these pericellular binding sites corresponded to functional receptors, their properties were compared with those of previously well-characterized [125I]somatostatin binding sites present on neuronal processes on the same sections in the stratum radiatum of the CA1 of the hippocampus. Radiolabelled sections were analysed by densitometry using a Biocom image analysis system coupled with a Leitz orthoplan microscope. The linear relationship between optical densities and radioactive standards allowed us to quantitate [125I]somatostatin-specific binding. Binding was time- and temperature-dependent, and saturable and specific in the arcuate nucleus as in the CA1 of the hippocampus. Saturation experiments indicated a single receptor population of binding sites with KD values of 0.2 +/- 0.1 nM in the arcuate nucleus and 0.6 +/- 0.4 nM in the CA1. In both structures, displacement curves obtained with somatostatin 14 and somatostatin 28 were monophasic, but shallow, while the somatostatin analogue SMS 201-995 induced a biphasic displacement, suggesting two populations of binding sites. In both regions binding was GTP-dependent. Desaturation procedures (in vivo by cysteamine and in vitro by preincubating with GTP) resulted in an increase in the number of measurable binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)