The object of the study was to examine the effect of varying the time of the evening meal on the pattern of 24-h intragastric acidity. Ten healthy subjects were studied; they ate regular meals throughout the day, but between 17.00 and 21.35 hours were separated into three groups. On three different days each group was fed the same dinner at either 17.15, 19.15, or 21.15 hours (early, standard or late). Variation in the evening meal's time caused significant changes in the pattern of acidity in the afternoon and evening, but did not affect 24-h intragastric acidity or nocturnal acidity. Integrated afternoon acidity (14.00 hours to dinner) was 69, 169 and 324 mmol.h/L when the subjects ate early, standard and late meals, respectively; evening acidity (dinner to midnight) was 235, 43 and 1 mmol.h/L with the three meals, respectively. The results suggest that, to control intragastric acidity, when the evening meal is eaten early (17.15 hours) dosing with an H2-antagonist should be after that meal, when eaten at the standard time (19.15 hours) dosing should be at bedtime, but when dinner is late (21.15 hours) the optimal regimen may involve dosing after lunch and also at bedtime.