Although it is well established that the analgesic effects of morphine are mediated by opioid receptors, previous studies have shown that some opioids additionally inhibit the uptake of serotonin and norepinephrine. The present investigation of a diverse group of opioids revealed that structurally identifiable subgroups inhibited the neuronal reuptake of these monoamines. Phenanthrene opioids with an oxygen bridge between C4 and C5, such as morphine and naloxone (group I), did not block norepinephrine or serotonin uptake, whereas phenanthrene opioids without the oxygen bridge and the C6-OH moiety, such as levorphanol and levomethorphan (group II), did inhibit uptake, as did nonphenanthrene opioids, such as d-propoxyphene and methadone (group III). Affinity at the mu opioid receptor correlated with antinociceptive potency (r = 0.87, P < .05). Although the antinociceptive activity of the "active enantiomers" of group II and III compounds also correlated with their affinity at the mu opioid receptor (r = 0.85, P = .007), additional consideration of serotonin uptake inhibiting activity (but not of norepinephrine uptake inhibiting activity) significantly improved the correlation between antinociceptive potency and the in vitro activity of these compounds (r = 0.915, P = .0017). Additionally, for group II and III (but not group I) compounds, smaller differences between enantiomers in antinociceptive potency than in mu receptor affinity were noted, presumably because of the contribution of uptake inhibition to the antinociceptive activity of group II and III compounds. Evidence also is provided suggesting a broader role for the combination of mu opioid affinity and 5-hydroxytryptamine uptake inhibition in the activity of other antinociceptive agents.