Two experiments were performed to assess the embryotoxicity of intravenously administered ytterbium chloride in golden hamsters: (1) 50, 75, 100, or 150 mg/kg ytterbium chloride were injected early on the 8th gestation day; (2) 100 mg/kg ytterbium chloride were administered at 4 different times during the critical period of organogenesis in this species (7th day, 4 PM; 8th day, 8 AM or 4 PM; 9th day, 8 AM). The pregnant females were killed on the 12th, 14th, or 15th gestation day and the frequency of resorptions was determined, and surviving offspring were studied for the presence of external and internal malformations as well as skeletal damage. Relatively few externally visible malformations and no significant internal malformations were produced. Damage to the skeletal system included fused and supernumerary ribs, and many different sites of poor ossification. Maximal susceptibility to these effects occurred on the 8th day.