Two analytical methods, radioreceptor assay and radioimmunoassay, for the determination of dihydroergotoxine have been developed. Antiserum, providing sufficient sensitivity for the radioimmunoassay, was produced by immunizing rabbits with D-lysergic acid coupled to bovine serum albumin. Radioreceptor assay utilizing dopamine receptor was carried out to determine dihydroergotoxine and its pharmacologically active metabolites in rabbit plasma, and the result was compared with that obtained by radioimmunoassay. The values obtained in both assays were almost identical; it was, therefore, assumed that the plasma concentrations of dihydroergotoxine determined by the present radioimmunoassay reflects the amount of unchanged drug and its active metabolites.