The effects of in utero exposure to the fescue endophyte Acremonium coenophialum on neonatal Cortisol, thyroid hormone, and progestagen concentrations were examined. Blood samples were collected at birth from 12 mares and foals exposed to the fescue endophyte A. coenophialum (E +; mean gestation, 358 days) and 6 mares and foals grazing endophyte-free (E -) fescue (mean gestation, 334 days). The plasma was assayed for tri-iodothyronine (T 3 ), thyroxine (T 4 ), reverse T 3 (rT 3 ), Cortisol, ACTH, and total progestagens by standard RIA. The maternal total progestagen concentration was significantly lower (p < 0.01; 4.7 ng/ml) in mares grazing E + fescue compared to mares grazing E-fescue (22.4 ng/ml). In the foals exposed in utero to endophyte, total progestagens, Cortisol, and T 3 concentrations were significantly lower (p < 0.01; 6.7 ± 2.2 vs. 42.4 ± 8.5 ng/ml progestagens; 145.3 ± 23.1 vs. 360.0 ± 21.4 ng/ml T 3 ; and 4.1 ± 2.1 vs. 59.5 ± 6 . 1 ng/ml Cortisol) than corresponding concentrations in foals not exposed to endophyte. ACTH concentrations in foals that died acutely after delivery were significantly lower at birth than values in foals that survived (37.6 ± 16.47 vs. 291.4 ± 69.8 ng/ml). Umbilical vein progestagen concentrations did not differ between treatment groups (19.7 ± 3.2 vs. 18.3 ±2.7 ng/ml). In foals that survived the natal period, there was a significant increase in Cortisol, T 3 , and progestagens from birth to 12 h of age. In foals that succumbed within the first 24 h, no increase in progestagens, T 3 , or Cortisol was noted. These results demonstrate that exposure of pregnant mares to A. coenophialum-infected tall fescue results in altered neonatal adrenocortical and thyroid hormone production. Placental steroidogenesis as evidenced by umbilical vein progestagen concentrations appears unaffected. Therefore, we suggest that prolonged gestation and fetal dysmaturity in fescue toxicoses results from abnormal adrenal and thyroid hormone production by the late-term fetus.

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