The cyclopian type of malformation occurred in lambs from ewes that were bred in August or early September while on particular ranges in Idaho. Incidence ranged from 1 to 25% in lambs in different herds, with wide variations from year to year. All the malformed lambs born at term came within the first 2 to 3 weeks of the lambing season. The relation between range plants and the disorder was studied in preliminary feeding trials from 1955 to 1959 with ewes and rats, in range grazing trials with ewes in 1959, in artificial feeding with dried Veratrum californicum from different areas for 30 days after breeding in 1960 and in artificial feeding with fresh green V. californicum for 5, 10, 15, 20 or 30 days from the date of breeding in 1961. The malformation was reproduced under the controlled experimental conditions by giving green V. californicum either fresh or dried. Degree of toxicity and teratogenic effect varied widely between different areas of the range. When ewes ate the plant during the first 10 days of pregnancy the fetus was not affected. If it was eaten up to the 15th day deformities of a cyclopian type occurred. When ingestion continued after the 15th day an abnormally large number of ewes had disturbed reproductive cycles and all foetuses obtained from those ewes before the disturbance had died and were severely malformed.-F. White.