The composition of pooled plaque fluid from exposed root surfaces of five population samples was determined before and at selected times (3, 7 and 15 min) after a 5 and 10% sucrose rinse. Subjects were 45-65 yr old, had exposed, sound root surfaces, and were grouped according to root caries status [caries-free (CF), no root caries history; caries-positive (CP), recorded root caries experience]. Samples were obtained only from the sound surfaces of the CF and CP subjects after overnight fasting. Plaque samples from each subgroup were pooled under mineral oil and maintained on ice. Plaque fluid was then isolated by centrifugation and analysed for organic acids and inorganic ions (ion chromatography), and pH (microelectrodes). From these data, the degree of saturation [DS(TM)] in plaque fluid with respect to tooth mineral (TM) was calculated. Before exposure to sucrose, plaque fluids from the CF and the CP subgroups had similar ionic compositions. These fluids were also found to be supersaturated with respect to tooth mineral. After exposure to sucrose, a rapid decrease in plaque fluid pH was observed, which corresponded primarily to lactic acid production. For all times examined, mean pH and DS(TM) values were lower and lactic acid concentrations were higher in the CP than the CF samples. Lower values of DS(TM) suggest that plaque fluid from CP subjects had a measurably greater cariogenic potential. Calcium concentrations also increased significantly and to comparable levels in all plaque fluid samples after sucrose exposure, despite lower acid production in CF samples.(ABSTRACT TRUNCATED AT 250 WORDS)