The XVIII North American Testes Workshop will be held in Seattle, Washington, on March 30th through April 2nd, 2005. The title theme for the meeting is Testicular Cell Dynamics and Endocrine Signaling. The theme and content of the meeting will make for a complementary contrast to the genomic and proteomic orientation of the previous Workshop of 2003. As was described at the previous meeting, the new profiling methodologies revealed many genes and gene products that were identified in the male reproductive system for the first time, creating an immense backlog of functional understanding. The challenge remained to determine how the larger cast of proteins subserves the main physiological roles of the testes, sperm production and androgen synthesis. The 2005 Workshop assembles a roster of speakers who are uniquely qualified to educate the attendees on advances in the field over the last two years. The Workshop will be organized into six sessions: genetics, development, the endocrine axis, steroidogenesis, spermatogenesis, and clinical correlates. Genetics will be represented in the keynote delivered by Dr. Eva Eicher of the Jackson Laboratory. Using mouse genetics, she will cut through the complexity of the downstream effectors of the sex-determining region of the Y chromosome (Sry), and clarify how the male body plan is encoded by the genome. The subsequent sessions will turn to endocrine signaling and paracrine cross talk between the germ and somatic cells of the testes. The final session on clinical correlates is intended to bring three perspectives on potential targets for drug intervention in the treatment of infertility and male contraceptive development. In addition to talks on fertilization and sperm chemotaxis, we will have Dr. Jon Jarrow of Johns Hopkins presenting data on the androgen microenvironment in the human testes. Differences in androgen metabolism unique to the human testes may have profound implications for our understanding of how spermatogenesis is regulated in men.