Oral administration of l-propoxyphene with d-propoxyphene enhances the analgesic activity of d-propoxyphene as expressed in the rat tail heat test. The combination of d- and l-propoxyphene at a dose of 10 mg/kg each was found to have activity in the analgesic assay equivalent to that observed with d-propoxyphene at a dose of 20 mg/kg. In the same test, l-propoxyphene at a dose of 40 mg/kg had no activity. Co-administration of equal amounts of l-propoxyphene with d-propoxyphene (10 mg/kg p.o.) results in an increase in circulating plasma levels of d-propoxyphene from 9 +/- 2 to 114 +/- 39 ng/ml 15 minutes after administration. The increase in plasma levels is accompanied by a proportional increase in the brain and lung levels with no significant change in the liver levels. When d-propoxyphene (4 mug/ml) was infused in the isolated perfused rat liver, over 98% of the drug was extracted in a single pass through the liver. When l-propoxyphene was added to the perfusate (4 mug/ml), the extraction of d-propoxyphene was decreased to less than 90%. These results indicate that l-propoxyphene increases the systemic availability of d-propoxyphene by altering the amount of d-propoxyphene extracted by the liver.