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1. The normal lipolytic activity (LA) of rat aorta shows considerable individual, age, and sex differences. 2. LA was determined by release of non-esterified fatty acids (NEFA) from lipaemic human serum. 3. Addition of eserine in concentrations inhibitory only to cholinesterases results in approximately 50 % inhibition of LA. 4. Low concentrations of tetra isopropylpyrophosphoramide (iso-OMPA) produced a mean inhibition of 93 %. Up to 100 % inhibition occurred in individual cases. 5. These results suggest that at least 50 % LA of rat aorta depends on non-specific cholinesterase (ChE) and the remainder possibly on an eserine-resistant, iso-OMPA- sensitive aliesterase. 6. Administration of iso-OMPA to living animals produced a fall in plasma NEFA and a reduced aortic LA (to 30 % of normal). ChE activity in cold microtome sections of aorta was completely inhibited. 7. Since no clinical or morphological changes could be observed in the iso-OMPA treated animals, its long term administration is apparently perfectly feasible.