The birth dates of neurons showing glutamic acid decarboxylase (GAD) immunoreactivity have been determined in mouse somatosensory cortex. Pregnant C57B1 mice received pulse injections of (3H)thymidine from E10 through E17 (EO being the day of mating). The distributions of thymidinelabeled, GAD-positive and nonimmunoreactive (non-GAD) cells as a function of depth under the pial surface were recorded in adult animals. The maximum rate of generation of GAD-positive neurons occurred at E14, whereas the generation of non-GAD neurons reached its maximum rate at E13. Except for those in layer I, GAD-positive neurons followed an inside-out sequence of positioning. GAD-positive neurons born at E12 and E13 were located in layers VI—IV. GAD-positive neurons born at E14 were found throughout the cortical thickness, with a maximum in layer IV. The GAD-positive neurons labeled after pulses at E15 or E16 or E17 were limited to the superficial strata, forming a band that became narrower as it moved toward the pial surface with increase in age of pulse labeling. GAD-positive neurons in layer I were generated at a constant rate during the whole embryonic period analyzed. Non-GAD neurons also followed an inside-out spatiotemporal gradient. Two partially overlapping phases were distinguished in non-GAD neurogenesis. During the first phase (from E12 to E14) neurons populating adult layers VI and V originated, while neurons located in layers IV through I were generated during the second phase (from E13 to E17). Since GAD-immunoreactive neurons form a heterogeneous population, we envisage further studies in order to test whether differences exist in birth dates among the classes.