Environmental conditions that impede heat dissipation and increase body temperature cause heat stress (HS). The study objective was to evaluate impacts of HS on the follicular phase of the estrous cycle. Postpubertal gilts (126.0 ± 21.6 kg) were orally administered altrenogest to synchronize estrus, and subjected to either 5 d of thermal-neutral (TN; 20.3 ± 0.5 °C; n = 6) or cyclical HS (25.4 - 31.9 °C; n = 6) conditions during the follicular phase preceding behavioral estrus. On d 5, blood samples were obtained, gilts were euthanized, and ovaries collected. Fluid from dominant follicles was aspirated and ovarian protein homogenates prepared for protein abundance analysis. HS decreased feed intake (22%; P = 0.03) and while plasma insulin levels did not differ, the insulin:feed intake ratio was increased 3-fold by HS (P = 0.02). Insulin receptor protein abundance was increased (29%; P < 0.01), but insulin receptor substrate 1, total and phosphorylated protein kinase B, superoxide dismutase 1, and acyloxyacyl hydrolase protein abundance were unaffected by HS (P > 0.05). Plasma and follicular fluid 17β-estradiol, progesterone, and lipopolysaccharide-binding protein concentrations as well as abundance of steroid acute regulatory protein, cytochrome P450 19A1, and multidrug resistance-associated protein 1 were not affected by HS (P > 0.05). HS increased estrogen sulfotransferase protein abundance (44%; P = 0.02), toll-like receptor 4 (36%; P = 0.05), and phosphorylated REL-associated protein (31%; P = 0.02). Regardless of treatment, toll-like receptor 4 protein was localized to mural granulosa cells in the porcine ovary. In conclusion, HS altered ovarian signaling in postpubertal gilts during their follicular phase in ways that likely contributes to seasonal infertility.