Developmental changes of soluble and particulate pyroglutamyl (pGlu)-peptidase I activities in the rat brain cortex and in the cerebellum are described in this work. The enzyme activity has been measured spectrofluorimetrically using pGlu-b-naphthylamide as substrate (in the presence and absence of EDTA and DTT, necessary activators of the enzyme) in both soluble and particulate fractions. In the soluble fraction of the cerebellum and brain cortex, pGlu-peptidase I activity is high in the perinatal period and decreases two or three folds subsequently, at a later stage in the cerebellum than in the brain cortex, reaching adult levels at the end of the first postnatal month. The decrease in the activity of pGlu-peptidase observed in this work coincides with increasing levels of brain thyroliberin concentration after the second postnatal week. The particulate pGlu-peptidase I activity, obtained after osmotic shock and high-salt treatment, shows less significant changes during brain development in the areas under study. It is suggested that cytosolic pGlu-peptidase I could play a part in the normal development of the rat central nervous system.