Reports of morphological differences between the brains of humans with different sexual orientation1 or gender identity2 have furthered speculation that such behaviours may result from hormonal or genetic influences on the developing brain. However, the causal chain may be reversed; sexual behaviour in adulthood may have caused the morphological differences. I report how adult sexual experience alters the appearance of rat motor neurons as revealed by Nissl staining, the same technique used in post-mortem human studies.