Two groups of surgical patients each comprising six individuals received an intrathecal injection of morphine 0.3 mg or meperidine 10 mg. Cerebrospinal fluid (CSF) and plasma were sampled frequently during a 6-h period and analyzed for morphine or meperidine. Maximum plasma morphine concentrations were found 5-10 min after injection, and averaged 4.5 +/- 1.1 ng.ml-1 (mean +/- SEM). Maximum CSF morphine concentrations were considerably higher than maximum plasma concentrations, 6410 +/- 1290 ng.ml-1. Maximum plasma concentrations of meperidine were also measured 5 or 10 min after injection and were low (36 +/- 9 ng.ml-1) compared with the maximum CSF concentrations (364 +/- 105 micrograms.ml-1). After a rapid initial decline for about 15 min after injection, the CSF concentrations decreased with a half-life of 89.8 +/- 16.1 min for morphine and 68.0 +/- 5.1 min for meperidine during the rest of the study period. The initial volume of distribution in CSF was similar for both drugs, or 22 +/- 8 ml for morphine and 18 +/- 5 ml for meperidine. After 6 h, 1.6 +/- 0.9% of the injected morphine dose and 0.41 +/- 0.09% of the meperidine dose remained in the initial volume of distribution. Large inter-individual differences in morphine and meperidine CSF kinetics existed, which may explain some of the reported individual differences in duration of effects. The disappearance of meperidine from CSF tended to be faster than that of morphine, which may be explained, in part, by the differences in lipid solubilities of the drugs.