The association of flow rate and biochemical and microbiological characteristics of saliva with diet was studied in 83 12-year-old children and in 127 adults (84 in the age group 35-44 years, 43 in the age group 65-74 years) living in rural and urban communities in Tanzania. No significant differences were observed between the salivary flow rates of the rural and urban subjects. The mean salivary flow rates were slightly lower in women than in men and significantly lower in the 12-year-old children than in the two older age groups (p < 0.05). The buffer effect was higher in the rural than the urban population. Further, it was lower in women than in men (p < 0.001). Salivary protein, IgG, and sialic acid concentrations were significantly higher in the rural than in the urban population, whereas amylase activity and IgA concentrations were lower. Microbiological studies showed mutans streptococci in 97% of the rural and in 91% of the urban subjects (n.s.). Lactobacilli grew significantly less often in the rural than urban samples (p < 0.01). Analyses of 24-hour dietary recalls showed that the rural and urban diets differed. The proteins in the rural diet were largely of plant origin, while in the urban diet animal proteins dominated. The rural diet contained also less sucrose but more fibre (mainly from grain) than the urban diet.(ABSTRACT TRUNCATED AT 250 WORDS)