Early events in an acute inflammatory response include the adherence of neutrophils (PMN) to capillary endothelial cells and the migration of these cells into tissues. This study was designed to determine if the opioid peptides beta-endorphin or met-enkephalin would induce a concentration-dependent increase in PMN adherence to serum-coated glass. Results show that PMN adherence is increased with both beta-endorphin and met-enkephalin and this increase may be partially blocked by the opiate antagonist naloxone. Binding of opioid peptides to the formyl peptide receptor was ruled out as a mechanism of increased adherence by showing that the opioids failed to block the binding of f-met-leu-phe-lys to PMN. These studies suggest that alterations in circulating opioid concentrations may modulate the adherence of PMN and thereby influence acute inflammatory reactions.