Two assay procedures are described for the quantitation of norantipyrine sulphate and norantipyrine glucuronide present in biological material. One, a selective acid hydrolytic assay procedure that affords the discriminative determination of both conjugates without prior separation, measures free norantipyrine by tlc-uv. The other is a tlc separation procedure for intact norantipyrine conjugates, which, in conjunction with radiolabelled material, derived from [3-14C]antipyrine, enables the direct quantitation of both conjugates independently. In man, about 25% of dose of antipyrine (1200 mg) was excreted as norantipyrine glucuronide within 48 h. The amount of norantipyrine sulphate was small. In the rat, norantipyrine sulphate represented about 15-20% of the dose of antipyrine (40-50 mg kg-1); no norantipyrine glucuronide was formed. Free norantipyrine has not been detected in urine of either species after antipyrine dosage. Pretreatment with 3-methylcholanthrene in the rat substantially enhanced excretion of norantipyrine sulphate, whereas induction with phenobarbitone was without effect on the microsomal N-demethylation of antipyrine. The lability of free norantipyrine was examined under different conditions and contrasted with the relative stability of norantipyrine conjugates. Two of the main degradation products of norantipyrine were identified as 4-phenylazo-norantipyrine and a 4,4'-bipyrazole-derivative.