Intercellular adhesion molecule-1 (ICAM-1) is thought to facilitate the recruitment and migration of monocytes/macrophages to sites of inflammation. Here we investigated whether the luteolytic effect of prolactin in the hypophysectomized rat is associated with the expression of ICAM-1. In addition, we examined the effect of exogenous testosterone (or its potential conversion to estradiol endogenously) on the corpus luteum to address recent speculation that ovarian steroids might augment luteal regression. Immature, 30-day-old rats were ovulated with eCG and hCG and then hypophysectomized; this resulted in a single cohort of persistent corpora lutea. The rats were assigned randomly into four treatment groups: vehicle treatment without or with testosterone (VEH-T4, VEH T4) and prolactin treatment without or with testosterone (PRL−T4, PRL T4). Corpora lutea of control rats exhibited minimal ICAM-1 staining and contained relatively few monocytes/macrophages. In contrast, corpora lutea of prolactin-treated rats exhibited prominent ICAM-1 staining and contained numerous monocytes/macrophages. Testosterone did not overtly affect ICAM-1 staining, numbers of monocytes/macrophages, or concentrations of plasma progestins (progesterone and 20α-dihydroprogesterone) in either VEH or prolactin treatment groups; notwithstanding, luteal weights increased significantly in response to testosterone in VEH T4 rats compared to VEH-T4 rats and prolactin-treated rats. We conclude that ICAM-1 expression and monocyte/macrophage accumulation are associated with prolactin-induced luteal regression in the rat and that these aspects are not influenced by testosterone.