AbstractA series of 49 normal male and female adult hamsters, nearly equally divided as to sex and 37 pregnant animals received ethionine subcutaneously or intraperitoneally. Normal animals were given 1.0, 0.7 or 0.4 mg per gram of weight per day over a period of 4 to 14 days. Pregnant animals received either 1.0 or 0.7 mg, beginning at different intervals during gestation: from the third to sixth; on the eighth, of from the tenth to twelfth days, the course ranging from two and one‐half to 15 days. Normal females were more sensitive than males. Among the latter, the 1.0 mg dose produced degeneration of exocrine tissue in ten days with no deaths, while in normal females, 0.7 mg gave similar results with little mortality or liver pathology. In pregnant females, the drug had adverse effects, vaginal bleeding and high mortality resulting when administered on the eighth day or later. From 11 survivors, only two litters were obtained, no damage being found in either maternal or neonatal pancreases. Pregnant animals started on the third to sixth day with the 0.7 mg dose tolerated it well. However, all but one of 17 expected litters died long before birth. The pancreases from the new‐born were normal with only slight damage found in the maternal pancreas. The route of administration was of no conseque