Using a specific radioimmunoassay we have investigated the distribution of octadecaneuropeptide (a putative endogenous ligand at the benzodiazepine receptor)-like immunoreactivity (ODN-IR) in rat brain and peripheral tissues. Highest concentrations in brain were found in the hypothalamus, cerebellum and substantia nigra. Significant concentrations of ODN-IR were found in all peripheral tissues studied and in plasma. Chromatographic analysis revealed several molecular forms; one major form, indistinguishable from the synthetic peptide, was found predominantly in peripheral tissues and in plasma, while another major form, of higher molecular weight, was found in brain, peripheral tissues and plasma. Although ODN-IR was present in the synaptosomal fraction, concentrations in the microsomal fraction were higher than for other neuropeptides studied.