A sensitive HPLC assay for measurement of the antipsychotic drug, olanzapine, in plasma has been developed. The assay has a limit of quantitation of 1 ng ml-1 in plasma and utilizes solid-phase extraction and electrochemical detection. The method provides a linear response for olanzapine over a concentration range of 1-100 ng ml-1 with coefficients of determination greater than 0.9912. The inter-assay precision was 15.9% at the limit of detection and ranged from 7.33% to 8.47% over the range of 5-100 ng ml-1. The intra-assay precision was in the range 0.97%-26.0%. The inter-assay accuracy ranged from 98.9 to 118% and the intra-assay accuracy ranged from 92.5% to 125% of the theoretical value. In addition, the assay was extended to measure the plasma levels of two metabolites of olanzapine, namely the N-desmethyl- and the 2-hydroxymethyl analogs. The utility of the assay was demonstrated following the administration of a single oral dose of 14C-olanzapine to rats where, at several time-points after dosing, the plasma was assayed for total radioactivity, levels of olanzapine, and the two metabolites. Olanzapine and two of its metabolites accounted for less than 50% of the total plasma radiocarbon; olanzapine accounting for approximately 39% at the Cmax, N-desmethyl for 5% and 2-hydroxymethyl for 8% respectively. The plasma elimination half-times for olanzapine and the two metabolites were approximately the same, ranging from 3.3 to 4.4 h.