Fetal maturation and equine parturition are not understood fully, although the adrenal and thyroid glands are thought to have regulatory roles. Thyroidectomized equine fetuses undergo prolonged gestation, and spontaneous diseases such as thyroid musculoskeletal disease and gestational fescue endophyte exposure are also associated with delayed parturition. Thyroid musculoskeletal disease is characterized by: histologically hyperplastic thyroid glands, chondro-osseous dysplasia and dysgenesis, angular limb deformity, low resting thyroxine and triiodothyronine concentrations, and lack of response to thyroid stimulating hormone. There are also similarities between foals born to mares grazing fescue grass infected with endophytes and foals with thyroid musculoskeletal disease (TH-MSD foals). It is thought that there may be an endocrine basis for the prolonged gestation observed in these disease states. The aim of the present study was to determine the endocrine competence of the adrenal gland in TH-MSD foals. Adrenocorticotrophic hormone (ACTH) response tests were used to compare the functional ability of the neonatal adrenal gland in healthy foals and TH-MSD foals. Basal thyroxine concentrations were significantly different between groups (P < 0.02): the thyroxine concentrations were lower in TH-MSD foals. After ACTH administration there was a significant effect of time (P < or = 0.001), but not treatment, on cortisol concentrations in foals. Thyroid hormone deficiency in TH-MSD foals did not significantly affect adrenal cortical secretion after ACTH administration. This finding indicates that thyroid function may play a major role in the timing of parturition either directly or indirectly via a mechanism other than by influencing adrenal responsiveness to ACTH.