1. Some effects of 6-azauracil and 5-fluorouracil on protein and RNA synthesis in Saccharomyces carlsbergensis were studied. 2. Both analogues caused a severe inhibition of RNA formation, whereas protein synthesis was much less affected. 3. Induced alpha-glucosidase formation was only slightly impaired. 4. Both analogues caused an inhibition of ribosome formation, although 5-fluorouracil was far more effective. 5. In the presence of the latter analogue abnormal RNA of high molecular weight and of more DNA-like base composition accumulated. On reincubation in medium free of analogue but containing uracil the abnormal RNA disappeared and was replaced by the normally sedimenting high-molecular-weight RNA species.