The pharmacokinetics and metabolism of RU 486 were characterized in 17 women who received a single dose of 600 mg of RU 486 for termination of an early unwanted pregnancy. Based on the clinical outcome and serum chorionic gonadotropin values, the subjects were divided into two groups: those who aborted completely (i.e. responders; N=13) and those who did not respond to RU 486 treatment (i.e. non-responders; N = 4). The serum levels of RU 486, the monodemethylated, didemethylated and hydroxylated metabolites of RU 486 were measured by HPLC preceded by column chromatography. There were no significant differences in the serum levels of RU 486 or its metabolites between the two groups. The serum concentrations of α1-acid glycoprotein, the binding protein for RU 486, were quantitated by immunoturbidimetry. The α1-acid glycoprotein concentrations were similar in responders and non-responders. The metabolism of RU 486 was also studied by fractionating extracts of serum pools of responders and nonresponders on thin-layer chromatography, and subsequent RIA analysis of the eluates of the sliced thin-layer chromatography. Spots with similar distribution and percentages of cross-reactivity were found in both groups on the chromatography; the results were also similar to those from a serum pool to which synthetic RU 486 and its three metabolites had been added. Hence it is concluded that failure to abort in response to RU 486 therapy is not associated with altered pharmacokinetics or metabolism of RU 486.