All the opiate-like peptides we tested (Met-enkephalin, (D-Ala2)-Met-enkephalin-NH2, beta-endorphin, (D-Ala2)-beta-endorphin, (D-Ala2)-alpha-endorphin, (D-Ala2)-gamma-endorphin) were capable of reducing distress vocalizations (DV's) in socially-isolated chicks when injected into the vicinity of the fourth ventricle in doses as low as 100 picomoles. All of these substances were at least as potent as equimolar doses of morphine sulfate. In general, DV's were a more sensitive measure of opiate-like peptide effects than reductions in body temperature. In a more limited study using peripheral injections, it was determined that (D-Ala2)-Met-enkephalin at doses of 400 nanomoles/kg, like morphine sulfate, was more effective in reducing DV's, than an equimolar dose of beta-endorphin. beta-endorphin was not as effective via a peripheral route as it was via central administration.