This study examined the effect of hyperthermia in late gestation on fetal thyroid structure. Mild heat stress (increase of ewe's rectal temperature to 40.4 degrees C) caused significant changes of fetal thyroid structure. Intra-follicular colloid area was significantly larger in the heat-stressed group (2,532 +/- 181 microns2) compared to a control group where rectal temperature of the ewe was within the normal range (1,660 +/- 188 microns2; p less than 0.01). Epithelial cell height was significantly reduced (2.5 +/- 0.19 vs. 4.8 +/- 0.26 microns; p less than 0.01). Fetal plasma glucose concentrations were significantly lower in the heat-stressed group (0.7 +/- 0.07 mmol/l) compared to the non-heated group (1.0 +/- 0.11 mmol/l; p less than 0.05). We suggest that the fetal thyroid gland in sheep is responsive to environmental temperature changes and is important in the neuroendocrine regulation of fetal body temperature. Fetal hypoglycaemia may also contribute to the changes in fetal thyroid gland by altering the peripheral utilization of thyroid hormone. Such changes, if prolonged, may contribute to the decreased rate of fetal growth and maturation associated with chronic heat stress in sheep.