Fitch & Denenberg have shown that manipulations of ovarian and testicu
lar hormones early in development can influence the adult size of the
corpus callosum in the rat. The human corpus callosum is highly variab
le in size and shape, but data are only now beginning to emerge on whe
ther sex steroids influence callosal differentiation in humans. I desc
ribe recent data from our own laboratory and suggest avenues for futur
e research.