The role of adipose tissue lipolysis in the postnatal amelioration of thermogenesis in 6- and 54-hr. -old domestic (D) and wild (W) piglets was studied. There were no significant differences (P > .05) in interscapular adipose tissue lipase (ATL) activity (Units/100 mg of tissue) of D vs. W piglets within age groups. ATL activity of W-54 piglets was significantly greater (P < .05) than that of W-6 piglets, whereas, the difference in ATL activity of D-6 and D-54 piglets was not statistically significant (P > .05). The total ATL activity (Units/100 g body weight) of W-6 vs. D-6 and W-54 vs. D-54 piglets was 1.6- and 1.3-fold greater, respectively.The relative changes, expressed as a percentage of zero-time values, in plasma free fatty acid (FFA) and glycerol concentrations at 4 and 8 min. post-norepinephrine injection were greater, with the exception of glycerol concentrations at 8 min. post-norepinephrine injection of 54-hr.-old piglets, in wild piglets as compared with domestics. The variability of the FFA and glycerol concentrations after norepinephrineinjection prevented definite conclusionsrelative to the relationship of sympathetic neural function to adipose tissue lipolysis in domestic and wild piglets. Histological studies which utilized both light and electron microscopyshowed no evidence for the presence of brown adipose tissue in wild piglets. These data are interpreted to indicate a greater total activity of adipose tissue lipase in wild piglets, and that the superior thermo-regulatory capabilityof wild piglets is due, in part, to a greater total activity of adipose tissue lipase.