Gastro-oesophageal reflux (GOR) occurs mainly during postcibal (PC) periods. The duration of PC gastric acidity and the incidence of GOR were analysed in 11 asymptomatic premature infants. GOR was studied during PC periods of 120 min and also during the following 120 min defined as fasting (FT) periods. These infants were subjected to simultaneous continuous gastric and oesophageal pH monitoring using a double-blind crossover technique. Two formulae with different fat contents (2.6 vs 3.6 g/100 ml) and different carbohydrate concentrations (8.1 vs 7.3 g/100 ml; malto-dextrin 2.8 vs 1.9 g/100 ml) were given. Gastric acidity (pH less than 4) in the PC periods lasted significantly (P less than 0.001) longer (68 min +/- 10) with the low fat/high malto-dextrin formula versus 43 min +/- 11 with the high fat/low malto-dextrin formula. Oesophageal pH monitoring data were within normal limits for the total investigation time in all infants. During PC periods acid GOR was detected more frequently in the group with a low fat formula. More PC GOR was recorded when the gastric acidity time was longer.