To test the Szego hypothesis of increased beta-glucuronidase and acid phosphatase activities in hormone target tissues. The presence of beta-glucuronidase and acid phosphatase activities in nuclear subcellular fractions obtained from decidual (implantation site) and stromal (nonimplantation zone) tissues was demonstrated by both biochemical measurements and ultramicrographic analysis utilizing a histochemical reaction. Acid phosphatase was almost twice as abundant in nuclei and lysosomes of epithelial cells (implantation sites), and beta-glucuronidase also was significantly more active in nuclei from epithelial and decidual tissues than in nonimplantation tissue. Our results, utilizing the implantation process as experimental model, support the Szego hypothesis of the lysosomal role in hormonal mechanisms of action.