Bovine CL were collected on days 5, 10, and 15 of the oestrous cycle following synchronised oestrus. In addition, CL were collected following prostaglandin-induced luteolysis. In situ hybridisation detected luteal expression of IGF-I, -II and the type 1 IGF receptor mRNA throughout the oestrous cycle. IGF-I mRNA concentrations varied significantly during the cycle, increasing from low levels in the early luteal phase (day 5) to high levels in the late luteal phase (day 15). Concentrations were maximal 48 hours after exogenous prostaglandin. In contrast, there was no significant effect of day of the oestrous cycle on IGF-II and the type 1 IGF receptor mRNA concentrations in the corpus luteum. IGF-II mRNA expression was localised to a subset of steroidogenic luteal cells and was also associated with cells of the luteal vasculature. In addition, the relative abundance of IGF-II mRNA was greater than that of IGF-I mRNA. mRNA encoding the type 1 IGF receptor was widely expressed, in a pattern suggestive of large and small luteal cell expression. The actions of the IGFs are modulated by their association with members of a family of IGF-specific binding proteins (IGFBPs), which regulate the transport of IGFs and their presentation to specific receptors. In situ hybridisation detected mRNA encoding IGFBP-2, -3, and -4 in the bovine corpus luteum throughout the luteal phase. IGFBP-2 and -4 mRNA concentrations were low within the corpus luteum, and showed no temporal variation. In addition, a subset of large vessels in the periphery of the CL showed moderate to intense hybridisation for IGFBP-2 mRNA. IGFBP-3 mRNA concentrations were high throughout the luteal phase, and expression was localised to small luteal cells and cells of the luteal vasculature. These data demonstrate for the first time that the bovine CL is a site of IGF production, reception and regulation, and further support the hypothesis that the IGF system is important in regulating luteal function in the cow.