Caloric or dietary restriction is known to be protective against cancer in humans and in mice but the mechanism is uncertain. Given that somatic mutations are important in carcinogenesis, dietary restriction may act by changing mutation rates. Indeed, previous studies have shown that reductions in caloric intake during development or in adult life make mice less susceptible to high doses of mutagens. In these studies there have been hints that the spontaneous mutant frequency may also be reduced, but no significant decrease has been observed save in one study of very old mice. Since the spontaneous mutant frequency is already low, reductions from this level require the use of much larger sample sizes than usual and larger than those used in the previous studies. As pre-existing mutations cannot be eliminated, it is necessary to reduce the dietary intake over a period of time when a substantial proportion of spontaneous mutations arise in order to see an effect. To overcome such problems, the dietary restriction in this study was applied during the time of the highest mutation rate, early development, and many more than the usual number of animals were studied. SWR female mice were crossed with Muta(TM)Mouse males to obtain F(1) progeny for analysis of mutant frequency. At conception, the dams were put into two groups, one that was fed ad libitum and another which was fed 80% of the ad libitum diet. Pups were killed at birth, DNA was extracted from the whole animal and used to measure the mutant frequencies of the mice at the cII locus. Although the weights of the pups from dams whose diet was restricted were significantly less than those of the ad libitum mice (P = 0.003), the litter sizes in the two groups were approximately the same and did not differ significantly (P = 0.13). There was no significant difference in the mutant frequencies in the dietarily restricted and ad libitum groups (P = 0.43). In addition, there was no significant correlation between the weights of the pups and their mutant frequency in either the ad libitum or dietarily restricted groups (r(2) = 0.14 and r(2) = 0.024). No difference was observed in mutant frequency between the ad libitum and dietarily restricted mice from litters of the same size (P = 0.61). These results indicate that the protective effect of dietary restriction on cancer rates is not mediated by an alteration in the spontaneous rate of mutation but rather by another mechanism, such as its effect on induced mutation.