A simple and rapid method for measuring neuroleptic drugs in plasma by the estimation of dopamine receptor blocking activity is described. The assay has the advantages that it can be used without modification for all neuroleptic drugs and measures both parent compound as well as pharmacologically active metabolites. Intra-assay and inter-assay coefficients of variation are 20% and 30% respectively and sensitivity is adequate to measure drug activity in plasma from patients treated with oral neuroleptics but not for those receiving depot injections. Preliminary clinical investigations indicate that the ranges of dopamine receptor blocking activity in plasma vary for different drugs. Interference in the assay has been seen with some samples from patients receiving amitriptyline and nortriptyline, but not with other psychotropic drugs. The receptor assay is considered suitable for routine monitoring of neuroleptic drug therapy.