The aim of this study was to investigate the effects of some intrinsic and non-milk extrinsic sugars on plaque pH. Three fruits (apple, orange and banana) in three different physical forms (whole, homogenized and juice) were tested. Ten adult volunteers refrained from brushing their teeth for 48 hours and did not eat or drink for at least 2 1/2 hours before each test. Measurements of plaque pH were made at baseline to determine the resting plaque pH and at time intervals of 3, 7, 11, 15, 19, 23 and 27 minutes following a 1-minute rinse/chew of each test fruit. A pooled sample of plaque was removed from the buccal surfaces of six teeth representing all four quadrants of the mouth and thoroughly mixed with 20 microliters of distilled water. Plaque pH was determined using a micro-glass combination electrode. Data were compared with plaque pH changes resulting from rinsing with 10% sucrose solution. Analysis of pH minima and 'area under baseline pH', using ANOVA and multiple Turkey tests, indicated that there was no significant difference in any of the parameters between the different forms of preparation within one fruit, except for the minimum pH of whole orange and orange juice (P < 0.05). The results showed that homogenization of the fruits had little effect on acidogenicity even though the intrinsic sugars had been converted to extrinsic sugars. Both whole and mashed banana were of similar acidogenicity to 10% sucrose. Therefore it was concluded that there was no significant difference in the acidogenic potential between intrinsic sugars and extrinsic sugars derived from fruits.