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The growth of the body and central nervous system and the emergence of stereotyped behaviour have been studied in male and female rats during the first 24 days of life. The effects of daily injections of equine gonadotrophin on these measures have also been investigated.

The weight of the body and of the central nervous system was significantly less in the female than in the male. The daily administration of 10 i.u. of equine gonadotrophin was without effect on either.

The movements of the trunk and limbs concerned in the body-righting reflex became coordinated more slowly in the gonadotrophin-injected animals than in their litter-mate controls.

At 15 days old, male rats were able to right in mid-air more successfully than litter-mate females.

The placing reflex appeared earlier in the male than in the female. Its appearance was accelerated in the females given gonadotrophin, but not in the males.

In the ventral funiculus of the spinal cord of 24-day-old experimental animals, the axis cylinders occupied more space relative to that occupied by myelin than did those of the controls. The total amount of myelin present was unchanged. There was no sex difference in the progress of myelination in the spinal cord.

The significance of these findings in relation to the secretion of sex hormones is discussed. It is suggested that the secretion of androgen may be responsible for an acceleration of nervous maturation.