Abstract The intravenous LD 50 of the hypothalamic tetradecapeptide somatostatin was determined in mice and rats of both sexes. It was found that somatostatin has a sex-related toxicity both in mice and rats. The interaction of low and high doses of peptide with the LD 50s of two central nervous system (CNS) drugs, namely pentobarbital and strychnine, was also studied in male and female mice. Differential effects were observed as follows: a very low dose (0.1 mg/kg) of somatostatin does not affect the toxicity of these compounds, whereas the injection of l mg/kg of peptide increased pentobarbital toxicity and decreased the toxicity of strychnine. However, an increase in the toxicity of both substances was obtained with very high non-lethal doses of peptide (20 and 30 mg/kg, in female and male mice, respectively).