T he original definition of the once obsolete word hormesis came to us from pharmacology, and meant a stimulation brought about by a low-level exposure to a substance that was toxic at high levels. In recent times, however, the word has been resurrected and the definition has been modified to refer not only to a stimulatory effect but also to a beneficial effect. In other words, hormesis now connotes a value judgment whereby a low dose of a noxious substance is supposedly good. Although one cannot deny that hormetic effects can occur with pharmacological agents, the situation is much less clear with ioniz-ing radiations, which produce random lesions within cells. The amount of energy deposited by low doses of radiation is just too small to bring about the physiological effects that could lead to stimulation. The reason for this, of course, is that Avogadro's number is so large that, even though the molar concentration of, say, an enzyme in a cell is small, the cell still will have a very large number of identical molecules necessary to carry out its proper metabolic function, which thus will not be affected by the destruction of a small percentage of the molecules. Consequently, to account for the effects oflow-level radiation, it has been necessary to look for a system within thecell that not only is sensitive to radiation, but also is capable of magnifying an individual lesion so that it can have a physiological effect. The genetic apparatus, the genes and chromosomes in the nucleus, represents just such a target for radiation. Radiation can induce mutations, occasionally by inducing some random base changes, but mainly by breaking chromosomes, which then can result in the broken pieces being deleted or rearranged, and these effects can have a profound influence on the cell.

The usual experiment on the genetic effects of ionizing radiations, however, has shown that the effects induced, rather than being hormetic with a beneficial effect, are deleterious (1). This has been shown in innumerable experiments in mutation in which it has been found that radiation-induced mutations themselves, unlike spontaneous ones, are, indeed, usually deleterious. That this should be so is not surprising, in that all living organisms are the result of eons of evolution in which they have been selected to fit their proper ecological niches. Any random mutational change then would be expected to change this fine balance and decrease fitness. With ionizing radiation, in which most of the induced mutations are deletions, this is even more likely. The question of hormesis after somatic irradiation is even more problematical, in that the deleterious effects of radiation would be different in each cell and, somehow, in the absence of strong selection (these are low doses after all) the effects would have to be