Male Sprague-Dawley rats (CrL:CD(SD)BR) were maintained under barrier conditions at Charles River Breeding Laboratories (Wilmington MA) from August, 1975, to July, 1983. Animals were provided food and water ad libitum. Survival data from 8 completed cohorts of 100 animals each and one continuing cohort reveal a highly significant linear increase in median lifespan, yielding a 26% increase in this parameter for cohorts born over a period of less than six years. The biological factors responsi ble for this increase are not clear at present. Nevertheless, these results in outbred rats, taken in conjunction with previous observations of a trend towards increased longevity in inbred mice, indicate that the assumption of cohort equivalence underlying many cross-sectional studies of biological aging may not be valid. © 1984 Taylor & Francis Group, LLC.