Studies described in this thesis were performed to investigate associations among season, diet, pituitary pars intermedia dysfunction (PPID) and blood concentrations of adrenocorticotropic hormone (ACTH), insulin, glucose, and leptin in horses. In the first study, higher ACTH concentrations were detected in horses affected with PPID. A seasonal increase in plasma ACTH concentration was detected in the late summer and early fall, but PPID did not affect the timing or duration of this increase. Pasture grazing raised glucose and insulin concentrations with a peak in September, at the same time that horses had higher ACTH concentrations, and this convergence of risk factors may raise the risk of laminitis. All of the horses included in this study were from the same farm. The second study was performed to determine whether horses from different locations within the same region exhibited the same seasonal increase in ACTH concentrations. Results of this study indicate that the seasonal increase in plasma ACTH concentrations occurs in horses from different farms with varying management practices. The third study investigated the effects of season on plasma leptin concentrations in the horses from the first study. We hypothesized that higher leptin concentrations would be detected in advance of the seasonal increase in plasma ACTH concentrations. Results did not support our hypothesis because leptin concentrations increased after ACTH concentrations peaked in September. Our findings suggest that the seasonal increase in ACTH concentrations induced leptin resistance, which might facilitate weight gain in the autumn. Alternatively, leptin concentrations increased as a result of weight gain or change in body fat composition. In summary, season appears to signal upregulation of the hypothalamic-pituitary-adrenal axis in horses, in an effort to prepare for winter. This upregulation is retained in horses with PPID, a disorder associated with loss of dopaminergic inhibition to the pars intermedia of the pituitary. The seasonal rise in plasma ACTH concentrations is followed by an increase in leptin concentrations, which suggests the development of leptin resistance or an increase in adiposity.