An earlier study showed that frequent gum chewing may enhance parotid gland function and reduce the acidogenicity of dental plaque. The aim now was to determine whether these effects would be observed after a 2-week period of diet altered to increase masticatory effort, and secondarily to assess the effects of chewing gum on masseter muscle activity. Ten subjects took part in the first experiment. Saliva was collected before and after the diet change and the plaque pH response to a sucrose challenge was measured. Subjects completed 3-day diet histories and wore electromyographic (EMG) devices to record masseter activity. In the second experiment, 10 subjects wore EMG devices for 3 days to record masseter activity on three daily regimens: baseline (no gum chewing), hourly gum chewing (sugar-free gum chewed for 10 min every hour) and chewing five sticks of gum each for 20 min during the day. Data were analysed by paired t test or repeated-measures analysis of variance. For the first experiment, EMG data indicated significant increases in chewing activity (p < 0.05), although there were no changes in salivary flow rates or the plaque pH response to sucrose. The second experiment showed that total EMG activity increased significantly on both gum-chewing regimens (p < 0.01), the magnitude of the increase being greater for hourly chewing. Overall, masseter EMG activity was increased 41% by diet alteration, compared to increases of 96 and 152% on the five-stick and hourly gum-chewing regimens, respectively.