Summary.: The developmental progression of two endopeptidases, four exopeptidases, trypsin inhibitors and chymotrypsin inhibitors in rat embryos from fertilization to the fully formed foetus and beyond has been evaluated. Biochemical and histochemical procedures using the same chromogenic substrates were applied to study uteri at various stages of gestation, embryos from the 11 th day of gestation to term, and kidney, liver and spleen from the 18th day of gestation to the adult stage. Activities were found to be lower in isolated embryos than in pregnant uteri but rates of increase were comparable except in the case of cystinedi-β-naphthylamidase which showed a greater rate of increase in uteri. Enzyme activities in liver, kidney and spleen were relatively high during the late stages of gestation, decreased at birth and increased again in postnatal tissues. The rate of change as well as the absolute values differed for each enzyme as well as for each tissue. There is evidence that soluble enzymes assayed by biochemical techniques are synthesized before particle-bound enzymes measured by histochemical techniques. Electrophoretic mobility, metal ion requirements and enzyme localization indicate that several closely related but distinct enzymes take part in foetal development.