Substance P is the best-known member of the family of tachykinin (TAC) peptides. Its distribution in spinal ganglia and substantia gelatinosa of the dorsal horn initially suggested a role in the transmission of sensory information from the periphery to the central nervous system (Hokfelt et al. 1975). Subsequently, an extensive distribution in the limbic system and brain stem (Ljungdahl et al. 1978) implicated these peptides in integrative and other processes as well. Mammalian TAC peptides are derived from two separate genes, usually designated preprotachykinins A and B. Preprotachykinin A mRNA codes for substance P, neurokinin A (NKA), neuropeptide K (NPK), and neuropeptide γ (NKγ; Krause et al. 1990), and preprotachykinin B mRNA codes for neurokinin B (NKB). Thus, the rat central nervous system contains at least five TAC peptides (Arai and Emson, 1986; Takano et al. 1986; Takeda et al. 1990; Tatemoto et al. 1985). Although substance P has a well-established role in the regulation of luteinizing hormone–releasing hormone (LHRH) secretion and sexual behavior, reproductively relevant roles for other TAC peptides are only now being explored. Much of what we can infer about the functional roles of TAC peptides in the central nervous system is derived from immunohistochemical studies, but antibodies to substance P often cross-react with other TAC peptides. Therefore, we have used the term “TAC-immunoreactive” (TACir) to describe the labeling of cells using antibodies directed against these peptides.