AbstractDevelopmental alterations in motor activity were quantified for the 1st 7 postnatal months in the kitten. Motor activity measured with a stabilimeter was low and constant for the 1st 9 postnatal days, increased markedly until Day 14, and remained stable until Day 21. Locomotor activity measured in an open field decreased slightly during the 1st 3 weeks, increased markedly during the next 2 weeks, and then remained relatively constant until the 9th week. Activity increased again during the 3rd and 4th month and then declined until 7 months of age. We suggest that the marked increase in motor activity during the 2nd week reflects development of visual and auditory systems whereas the increase in locomotor activity during the 4th and 5th weeks represents maturation of neural systems concerned with motor control. The underlying causes of the final activity changes probably represent the maturation of many neural systems and concomitant development of adult behavior in the cat.