The variability among subjects of the hepatic activities of O-sulphotransferase towards dopamine, p-nitrophenol, testosterone and ethinyloestradiol and of N-sulphotransferase with 1,2,3,4-tetrahydroisoquinoline (TIQ) as substrate is described. The rates of testosterone and TIQ sulphation were higher in men than women whereas those of ethinyloestradiol, dopamine and p-nitrophenol were similar in both sexes. The sulphotransferase activities towards p-nitrophenol and dopamine were positively skewed whereas those towards ethinyloestradiol approached normality. The coefficients of variations for the sulphotransferase activities ranged between 34% and 62% indicating a considerable variability among subjects. The rates of dopamine-, TIQ- and p-nitrophenol-sulphation were measured in the mucosa of the human intestine, and the duodenum/liver ratios were 10, 0.9 and 0.1, respectively. Thus the contribution of the intestine in the sulphation of xenobiotics is substrate dependent.