Sprague-Dawley rats were subjected to a 56-day caries test regimen when they were infected with S. mutans strain 6715 or 10449 at 18, 19, 20, 21, 22, 23 or 24 days of age. Results indicate that there was a decrease in mean caries scores as the age of inoculation increases. Inoculation of animals beyond 22 days of age may not produce caries scores significantly greater than uninoculated animals, suggesting that the time span of maximum caries susceptibility is shorter than previously thought. The two strains of S. mutans produced similar levels of caries. The study suggests that while the strain of microorganism inoculated may account for some of the variability in animal caries studies, the age of inoculation appears to be more important.