Microbial succession, experimental cariogenicity, and sucrose metabolism were examined in dental plaques which developed on sterile bovine enamel inserts in acrylic palatal appliances. The appliances were worn for a period of 14 days by 10 caries-free and 10 caries-susceptible human volunteers. Three of six enamel inserts on each appliance were exposed extraorally to 10% sucrose in 0.85% saline six times a day, and three were exposed simultaneously to 0.85% saline as a control environment. The responses of the plaques to the high-sucrose environment in both caries status populations were compared. In all plaques, exposure to 10% sucrose stimulated the succession of Veillonella spp., Lactobacillus spp., Streptococcus salivarius, and, to a lesser extent, Streptococcus mutans and a decline in levels of Streptococcus sanguis, Neisseria spp., and gram-negative anaerobic rods. Plaques from caries-free mouths, in contrast to those from caries-susceptible mouths, harbored higher levels of Veillonella spp., gram-negative anaerobic rods, and Neisseria spp. and lower levels of Lactobacillus spp. Sucrose-exposed plaques from caries-free mouths also induced less enamel microhardness changes and formed less lactic acid from [14C]sucrose during a 60-min incubation at 37 degrees C than did comparable plaques from caries-susceptible mouths. The experiments revealed consistent differences in the ecological response to a cariogenic substrate environment in plaques from the two populations, with plaques from caries-free subjects exhibiting less cariogenic potential than those from caries-susceptible subjects.