Ciliary neurotrophic factor (CNTF) has potent survival-promoting effects on motoneurons in vitro and in vivo. We examined knockout mice with null mutations of the gene for either CNTF itself or the alpha-subunit of the CNTF receptor (CNTFRalpha) to assess whether CNTF and/or its receptors are involved in the development of a sexually dimorphic neuromuscular system. Male rodents have many more motoneurons in the spinal nucleus of the bulbocavernosus (SNB) than do females. This sex difference is caused by hormone-regulated death of SNB motoneurons and their target muscles. Sexual dimorphism of SNB motoneuron number developed completely normally in CNTF knockout (CNTF -/-) mice. In contrast, a sex difference in the SNB was absent in CNTFRalpha -/- animals: male mice lacking a functional CNTF alpha-receptor had fewer than half as many SNB motoneurons than did wild-type males and no more than did their female counterparts. Size of the bulbocavernosus and levator ani muscles, the main targets of SNB motoneurons, was not affected in either CNTF or CNTFRalpha knockout males. These observations suggest that signaling through the CNTF receptor is involved in sexually dimorphic development of SNB motoneuron number and that target muscle survival per se is not sufficient to ensure motoneuron survival in this system. In addition, our observations are consistent with the suggestion that CNTF itself is not the only endogenous ligand for the CNTF receptor. A second, as yet unknown, ligand may be important for neural development, including sexually dimorphic motoneuron development.