AbstractGuinea pigs were exposed to hyperthermia for 1 hr once or twice on day 11, 12, 13, or 14 (E11–E14) of pregnancy. The mean rectal temperatures were elevated by 3.4°C–4.0°C. This treatment resulted in a marked elevation of rates of resorption and developmental defects in embryos examined at day E23. The defects observed were those affecting the neural tube (NTD) (exencephaly, encephaloceles, and microphthalmia), kyphosis/scoliosis, branchial arch defects, and pericardial edema. Embryos with NTD and kyphosis/scoliosis have not been found among newborn guinea pigs to date following maternal heat exposure on days E12–E14. It appears that embryos with these defects are filtered out by resorption or abortion by days E30–E35. © 1992 Wiley