Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) innervate the perineal muscles, bulbocavernosus (BC), and levator ani (LA). Testosterone regulates the survival of SNB motoneurons and BC/LA muscles during perinatal life. Previous findings suggest that effects of testosterone on this system may be mediated by trophic factors-in particular, by a factor acting through the ciliary neurotrophic factor ␣-receptor (CNTFR␣). To test the role of CNTFR␣ in the response of the developing SNB system to testosterone, CNTFR␣ ؉/؉ and ؊/؊ mice were treated with testosterone propionate (TP) or oil during late embryonic development. BC/LA muscle size and SNB motoneuron number were evaluated on the day of birth. Large sex differences in BC and LA muscle size were present in newborn mice of both genotypes, but muscle volumes were reduced in CNTFR␣ ؊/؊ animals relative to same-sex, wild-type controls. Prenatal testosterone treatment completely eliminated the sex difference in BC/LA muscle size in wild-type animals, and eliminated the effect of the CNTFR␣ gene deletion on muscle size in males. However, the effect of TP treatment on BC and LA muscle sizes was blunted in CNTFR␣ ؊/؊ females. SNB motoneuron number was sexually dimorphic in oil-treated, wild-type mice. In contrast, there was no sex difference in SNB motoneuron number in oil-treated, CNTFR␣ knockout mice. Prenatal treatment with testosterone did not increase SNB motoneuron number in CNTFR␣ ؊/؊ mice, but also did not significantly increase SNB motoneuron number in newborn wild-type animals. These findings confirm the absence of a sex difference in SNB motoneuron number in CNTFR␣ ؊/؊ mice. Moreover, the CNTFR␣ gene deletion influences perineal muscle development and the response of the perineal muscles to testosterone. Prenatal TP treatment of CNTFR␣ ؊/؊ males overcomes the effects of the gene deletion on the BC and LA muscles without a concomitant effect on SNB motoneuron number.