The author outlines some of the intrauterine influences on neonatal behavior. Sex hormones used therapeutically in pregnancy may critically alter the genotype's sexual expression as well as its behavior. Dietary inadequacy in early fetal life may change the number and size of the infant's brain cells. Drug abuse by the mother produces chromosomal flaws as well as withdrawal symptoms in the newborn. Tranquilizers and premedications given mothers during delivery affect the neonate's initial weight gain and his response to nursing and early learning tasks.Abstract Teaser