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The hormone-sensitive lipase of rat epididymal adipose tissue is tightly bound to the endogenous tissue lipids which float during centrifugation of homogenates. This endogenous lipid appears to provide the enzyme with saturating levels of triglyceride substrate. A method is described for extracting virtually all of this endogenous substrate with ether, after which about 25% of the original hormone-sensitive enzyme activity can be recovered in an insoluble fraction that now sediments. When assayed with triolein-C14, the activity of this fraction prepared from tissues incubated with epinephrine is twice that of preparations from tissues incubated without hormone.The activity toward monoglycerides and diglycerides, both in the whole homogenate and in the ether-extracted preparation, is more than an order of magnitude greater than that toward triglycerides at equal substrate concentrations. Recovery of activity toward lower glycerides was excellent in the ether-extracted preparations, occasionally almost quantitative. Activity against monoolein or diolein was not affected by preincubation of the tissue with epinephrine.The properties of the enzymes in the ether-extracted preparation suggest that the hormone-sensitive triglyceride lipase activity is distinct from the monoglyceride and diglyceride lipase activities. Whether the latter two are referable to the same or different enzymes cannot yet be decided.