Two hundred and fourteen normal offspring from 45 litters of mice were studied. Daily examinations employing a spectrum of reflexologic tests as indicators of neurologic development were followed from birth to 4 weeks of age. Five periods of neurologic development were found, each period having reflex responses characteristic of the behaviour and neurologic development of that age. The periods were designatedperinatal (birth to 3 days),neonatal (3 to 9 days),postnatal transition (9 to 15 days),postnatal infantile orpre-juvenile (15 to 26 days), andjuvenile (from 26 days until sexual maturity). These findings were correlated with data from previous studies, and it was confirmed that the end of the Postnatal transition period is the most critical period of development, when all aspects of neural maturation suddenly reach adultlike perfection. It was concluded that reflex tests are useful in assessing the degree of neural maturation in the developing mouse, and are reliable indicators of normal development.