Gyromitrin, the main poison of the false morel Gyromitra esculenta, causes enlargement of the liver in rats. The content of triglycerides and total cholesterol were increased in the liver homogenate and microsomal fraction but not in the mitochondrial fraction. On the other hand the content of phospholipids was found to be decreased. The enhanced incorporation of [14c]-acetate into the lipids of rat livers after gyromitrin indicates an increased rate of lipids synthesis. The diminution of the phospholipids is caused mainly by a loss of phosphatidyl ethanolamine (PE) although after a short pulse with [14C]-ethanolamine the specific radioactivity of the isolated phosphatidyl ethanolamine corresponded to that of the controls. Since the content of phosphatidylcholine (PC) remained unchanged, the PC/PE-ratio was shifted from 2.3 to 4.5. These effects were reversible within 48 h. Inhibition of the synthesis of highly unsaturated fatty acids by the microsomal monoxygenase system is discussed as the primary reason for the above observations this inhibition possibly being due to active metabolites of gyromitrin which destroy the monoxygenases.