The contribution to acidification of the stomach contents of pigs by hydrochloric acid secretion or by lactic acid produced by fermentation was studied in fifteen suckling pigs from six litters born and reared either in a 'conventional' environment or in an isolated 'clean' environment. Sequential samples of stomach contents obtained during periods of up to 24 h were analysed for their chloride and lactic acid contents, pH and total titratable acidity. These values gave a measure of organic and inorganic acids respectively. 2. Six pigs from two litters born and reared in a 'clean' environment had acid secretion in the stomach at 2 d of age, and the concentrations of lactic acid in stomach contents remained low (0-40 mmol/l) throughout the suckling period. 3. Eight pigs from three litters born and reared in a 'conventional' environment, and a ninth pig born in this environment but moved to the 'clean' environment at 24 h of age, had lactic acid in concentrations of up to 250 mmol/l in stomach contents within the 1st week of life. The pattern of lactic acid production (and hence the acidity of stomach contents) was governed by frequency of suckling. 4. Both between- and within-litter variation in the age of onset of HC1 secretion was evident in the group reared in a 'conventional' environment, and when HC1 secretion did occur it was usually accompanied by a reduction in lactic acid production. 5. It is concluded: (1) that the environment at birth is important in determining the fermentative ability of the stomach flora; (2) that if lactic acid is produced in large amounts in the stomach, it may partly or completely inhibit acidification by HC1.
