The appearance of sex-related differences in the excretion of sulphur derivatives of hexachlorobenzene (HCB) and pentachloronitrobenzene (PCNB) was studied in vitro and in vivo. Sexually immature rats given HCB showed initially no differences in the excretion of N-acetyl-S-(pentachlorophenyl)cysteine, but 5-8 days after weaning the urinary levels of the sulphur derivative began to increase in females until a 10-fold difference between both sexes was established. The studies in vitro and the analysis of tissues after in vivo administration of PCNB showed that conjugation with glutathione and hydrolysis of the conjugates to yield free pentachlorothiophenol do not present sex-related differences. These data tend to reinforce the view that an active renal secretory mechanism probably induced by estrogens during sexual maturation is responsible for the highly efficient excretion of sulphur derivatives of HCB and PCNB by female rats.