A radioreceptor assay for chlorpromazine in serum, which is based on binding to dopamine receptors, is described. This method has been postulated to measure all active metabolites as well as the parent drug. We have compared this method with an HPLC method for chlorpromazine. Dopamine-blocking activity, measured in serum samples from schizophrenic patients receiving chlorpromazine, was 1·85–9·1 times higher than serum chlorpromazine level measured by HPLC. The correlation between the two methods was 0·75. Dopamine-blocking activity was related more closely to dose of drug and to serum prolactin level than was serum chlorpromazine level measured by HPLC.
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