The quality of rendered leaf fat from mature breeder fowl which contained varying amounts of tocopherol due to in vivo dietary supplementation or due to direct in vitro addition was investigated. Treatments were dietary supplementation with d-alpha-tocopheryl acetate at levels of 0, 0.025, 0.050, 0.075 and 0.100% (0 to 1360 I.U./kg. feed) for 3 weeks pre-slaughter and in vitro d-alpha-tocopherol addition of 0, 10.74, 21.48, 32.22 and 42.96 (xg./ml. of rendered fat (0 to 6.4 I.U./100 ml. fat) of fowl fed 3 weeks on a diet containing no tocopherol supplementation. Parameters used to estimate fat quality included AOM stability time, free fatty acid content, initial peroxide value, and moisture and volatile matter content. Tocopherol content of the rendered fat was also determined.Fat tocopherol content was found to be significantly related to AOM stability time (r = 0.93, P < 0.01). The relationships of amount of in vivo or in vitro supplementation to the amount of tocopherol recovered in the fat and to AOM stability time were linear, positive and highly significant. The in vivo and in vitro methods of supplementation had significantly (P < 0.01) different effects on fat tocopherol content. Significantly less tocopherol was necessary to be present in the fat for in vivo vs. in vitro supplementation to obtain similar AOM stability times. However, in vivo supplementation required over 400 times the amount of tocopherol compared to in vitro supplementation to reach fat tocopherol levels where AOM stability times were similar. From these results in vitro addition of tocopherol appears to be the more economical and efficient procedure to improve quality and stability of rendered fowl fat.