Fluoxetine is a potent and selective inhibitor of the neuronal serotonin-uptake carrier and is a clinically effective antidepressant. Although fluoxetine is used therapeutically as the racemate, there appears to be a small but demonstrable stereospecificity associated with its interactions with the serotonin-uptake carrier. The goals of this study were to determine the absolute configurations of the enantiomers of fluoxetine and to examine whether the actions of fluoxetine in behavioral tests were enantiospecific. (S)-Fluoxetine was synthesized from (S)-(-)-3-chloro-1-phenylpropanol by sequential reaction with sodium iodide, methylamine, sodium hydride, and 4-fluorobenzotrifluoride. (S)-Fluoxetine is dextrorotatory (+1.60) in methanol, but is levorotatory (-10.85) in water. Fluoxetine enantiomers were derivatized with (R)-1-(1-naphthyl)ethyl isocyanate, and the resulting ureas were assayed by 1H NMR or HPLC to determine optical purities of the fluoxetine samples. Both enantiomers antagonized writhing in mice; following sc administration of (R)- and (S)-fluoxetine, ED50 values were 15.3 and 25.7 mg/kg, respectively. Moreover, both enantiomers potentiated a subthreshold analgesic dose (0.25 mg/kg) of morphine, and ED50 values were 3.6 and 5.7 mg/kg, respectively. Following ip administration to mice, the two stereoisomers antagonized p-chloroamphetamine-induced depletion of whole brain serotonin concentrations. ED50 values for (S)- and (R)-fluoxetine were 1.2 and 2.1 mg/kg, respectively. The two enantiomers decreased palatability-induced ingestion following ip administration to rats; (R)- and (S)-fluoxetine reduced saccharin-induced drinking with ED50 values of 6.1 and 4.9 mg/kg, respectively. Thus, in all biochemical and pharmacological studies to date, the eudismic ratio for the fluoxetine enantiomers is near unity.