Evidence is presented that all lipase activities present in the vascular and myocardial tissue from rat heart are regulated by product inhibition. Lipoprotein lipase activity, which plays a role in the uptake of circulating triglycerides, is determined by its reaction products, e.g. fatty acids and, predominantly, monoglycerides. Tissue acid and neutral lipase activities are regulated by product fatty acids and their coenzyme A (CoA) and carnitine ester derivatives. The order of potency is palmitoyl CoA approximately palmitoyl carnitine greater than palmitate for neutral lipase and palmitoyl carnitine greater than palmitoyl CoA palmitate for acid lipase activity. Product inhibition of extracellular and intracellular lipolytic processes warrants a close coupling between the supply of substrate fatty acids and the rate of fatty acid oxidation as determined by cardiac contractile activity. None of the lipases studied was directly affected by catabolic hormones (norepinephrine, glucagon) or their intracellular second messengers (cyclic AMP, protein kinase, Ca2+, calmodulin).