Butylated hydroxytoluene (BHT) was given in the feed to determine its effect on life span in genetically well-defined, barrier-derived BALB/c mice. Both sexes received 0.75% BHT for three different treatment periods: (A) 8 to 11 weeks of age; (B) for life, beginning at 11 weeks; (C) for life, beginning at 8 weeks of age. The control group (D) was untreated. All BHT treatment groups had mean survival times which exceeded that of controls. The order of survival was B greater than C greater than A greater than D (Males: 890, 832, 726, 684 days; Females: 875, 798, 759, 701 days). Most of the increases in mean survival time were related to a reduction in early deaths (350--600 days) in BHT-treated mice. The reason for the life-lengthening effect on BHT was not identified, but it may relate to alterations in specific disease incidences.