Neural salient serine/arginine rich protein 1 (NSSR1, alternatively SRp38) is a newly identified splicing factor that is highly expressed in neural and reproductive tissues. We showed that the expression of testicular NSSR1 increased significantly during mouse testes development. NSSR1 was mainly expressed in germ cells, but barely detected in Sertoli cells. Testicular NSSR1 was mostly phosphorylated and cytosolic in germ cells. In comparison, pituitary NSSR1 was mostly dephosphorylated and nuclear. In the cryptorchid testes, the dephosphorylated NSSR1 was significantly increased. RT-PCR analysis demonstrated that the alternative splicing of CREB and CREM genes was altered in the cryptorchid testes. In addition, CREB transcripts were associated with NSSR1 either in testes tissues or cultured GC-1 cells. Moreover, the studies with NSSR1 over-expression or silence demonstrated that NSSR1 promoted the exon 5 inclusion of CREB, indicating that NSSR1 is a new factor that regulates the alternative exon 5 inclusion of CREB transcripts. The findings for the first time provide the evidence indicating the potential importance of NSSR1 in testes development, spermatogenesis and cryptorchidism. Mol. Reprod. Dev. 74: 1363–1372, 2007. © 2007 Wiley-Liss, Inc.