The rat's corpus callosum is sexually dimorphic, with the male's being larger. In addition, giving rats extra stimulation in infancy via handling increases callosal area in males, but not in females. To determine if this dimorphism is testosterone-dependent, male pups were castrated on Day 1 of life while females received an injection of testosterone propionate (TP) on Day 4. Control males had sham surgery and control females received an injection of sesame oil. All animals were handled daily from birth until weaning. Animals were sacrificed at 110 days and a mid-sagittal section of the callosum was obtained. From this section measures of callosal area, perimeter, length, and 99 widths were derived. We verified our previous finding that the male callosum is larger than that of the female. Neonatal TP treatment masculinized the callosa of the females, but castration did not affect the males. TP treatment affected the width dimension of the callosum but not callosal length or brain weight. In a related study the synthetic estrogen DES did not increase callosal size for castrated males or for intact females, while the estrogen blocker, tamoxifen, had a defeminizing effect on females' callosa. These findings suggest that there is an estrogen-dependent active process of feminization of cortical tissue in the female brain.