lar aging, to which this model contributes, has grown. Apart from reports on work in this almost "classical" diploid cell system, the symposium presents studies using different biological systems with results that have been rewarding as information is obtained on patterns of change that are common to more than one experimental system. Indeed, in recent years much more has been learned about the fate of all different types of intermitotic and postmitotic cells in situ. The symposium has also presented contributions dealing, not directly with aging but with early ontogeny; such information on early developmental changes should certainly shed light on some of the mechanisms involved in aging. We are cognizant of the fact that environmental influences resulting from the complexities of modern civilization may have results that only occur much later, and profoundly affect the lifespan of the organism. There remain, of course, many unanswered questions. Whether there is "physiological" as opposed to "pathological" aging; whether "old" cultures living in unchanged, although not exhausted, medium, are degenerating, not aging; what is involved when "old" fragment cultures regenerate after excision by filling the wound with "young" cells; why some tumor cells in vivo as well as in vitro die while others live; all are questions eserving of our attention.

The three of us, working in different institutions but in the same city, were very aware of the differences between our diverse approaches to the biology of aging and our perceptions of the sub ject matter. However, three years ago we began to hold informal meetings to discuss our research. These meetings eventually be came more frequent and, with this association, we became increas ingly cognizant of the commonality of our research problems de spite our separate perspectives. The idea for this symposium, therefore, grew from our aware ness that the underlying problem of the biological basis for aging was a common denominator in our research. The papers presented here represent three areas of active investigation: cell divi sion, biological membranes and hormonal regulation. They are sub mitted with the expectations that a greater understanding of the role of each of these separate approaches will help clarify, not only the interrelationships between our fields of research, but more importantly, the biology of aging itself. ACKNOWLEDGEMENTS We would like to extend our sincere thanks for the interest and contributions of the companies listed below: Abbott Laboratories Hoffman-LaRoche Arthur D. Little McNeil Laboratories Bristol Laboratories Mead Johnson & Company Burroughs H'ellcome Co. Merck Sharp & Dohme Research Labs. Charles River Breeding Labs. Rom-Amer Pharmaceuticals Dow Chemical Co. Sandoz Pharmaceuticals E.R. Squibb & Sons, Inc. Schering Corp. Eli Lilly Research Laboratories Smith Kline Corp."