AbstractFertilized eggs of ring‐necked pheasants (Phasianus colchicus) were injected into the albumin or yolk with vehicle or graded doses of 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) (0.01, 0.1, 1, 10, 100, 1,000, 10,000, or 100,000 pg TCDD/g egg) on day 0 of embryonic development and toxicity was assessed in 1‐d‐old hatchlings and 28‐d‐old chicks. The most sensitive effect of in ovo TCDD exposure was induction of hepatic ethoxyresorufin‐O‐deethylase (EROD) activity in 1‐d‐old hatchlings. The ED50 for this response was 312 pg TCDD/g egg. Embryo mortality was the most sensitive sign of toxicity. The TCDD dose that caused 50 mortality above control (LD50) when injected into the egg albumin or yolk was 1,354 and 2,182 pg TCDD/g egg, respectively. At egg TCDD doses up to and including 1,000 pg TCDD/g egg, no effect was detected in 1‐d‐old hatchlings and 28‐d‐old chicks in body growth, organ weights, carcass morphometrics, incidence of edema, or incidence of histological alterations in the liver, spleen, heart, Bursa of Fabricius, or thymus. Egg TCDD doses as high as 1,000 pg TCDD/g egg also had no effect on cardiac morphometrics or incidence of cardiac malformations in 1‐d‐old hatchlings, or on antibody‐mediated immunity in 28‐d‐old chicks. We conclude that embryo mortality is the most sensitive sign of TCDD toxicity in the ring‐necked pheasant following in ovo exposure. The ring‐necked pheasant embryo is less sensitive than the chicken (Gallus domesticus) embryo and more sensitive than the eastern blu
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