Factors regulating somatic growth postnatally have been proposed to affect brain growth and subsequent function when applied prior to birth. This study pertains to the role of growth hormone administered to pregnant rats and the subsequent growth of the progeny. The results showed a significant increase in brain weight and cortical neurone number as determined by the incorporation of labelled thymidine into DNA and subsequent autoradiography. At maturity, learning performance on a series of conditional discrimination tasks was found to be enhanced.