We have undertaken to study the stereoselective disposition of (R,S)-tocainide, a new antiarrhythmic agent. To determine the enantiomeric composition of tocainide extracted from urine or blood serum, the drug was converted to diastereomeric derivatives by reaction with (S)-alpha-methoxy-alpha-trifluoromethylphenylacetyl chloride, and the diastereomers were separated by gas chromatography. The stereoselective excretion of tocainide in the 24-hr urine of male and female rats and mice was studied after 20-mg/kg ip doses of the racemic drug. The (R)/(S) ratio of excreted drug was greater than 1.0 in mice and less 1.0 in rats. Sex differences were also found. Pretreatment with phenobarbital significantly reduced the excretion of the (S) isomer in male and female mice and in male rats, and significantly reduced the excretion of the (R) enantiomer in male rats only. The (R)/(S) ratio of circulating tocainide in the serum of patients receiving the drug was significantly different from 1.0 in several samples analyzed. The reduction of 2-oxopropiono-2',6'-xylidide, the product of the oxidative deamination of tocainide, to 2-hydroxypropiono-2',6'-xylidide, a known circulating metabolite of tocainide in humans, was catalyzed by rabbit- and rat-liver cytosol, and the reduction proceeded with high stereoselectivity to give predominantly or exclusively the (S)-alcohol.