Exposure to opioid drugs (e.g., morphine) in vivo has been shown to suppress natural killer cell activity. However, the effects of in vivo exposure to opioids on cytotoxic T lymphocyte (CTL) activity has not been investigated. The administration of morphine (50.0 mg/kg, sc) to alloimmunized mice for 11 days resulted in a significant decrease in peritoneal and splenic CTL activity. Moreover, the intracellular content of serine esterases and esterase release by CD8+ effector cells from chronic morphine-treated mice was reduced compared to that of effector cells from vehicle-treated controls. In addition, the CD8+ cAMP response to alloantigen was diminished compared to CD(8+)-enriched cells from vehicle-treated animals. However, conjugate formation between effector and target and subsequent killing of target by effector cells did not reveal significant differences between vehicle- and chronic morphine-treated animals. Serum corticosterone and dehydroepiandrosterone levels were significantly lower in the chronic morphine-treated animals while proopiomelanocortin gene expression (exon 3) in splenic lymphocytes did not correlate with morphine-mediated suppression of CTL activity. These results indicate that CTL activity is sensitive to chronic morphine exposure, implicating opioids as important cofactors during viral infections in suppressing cell-mediated immunity.