The fermentation of [1-13C] glucose and [3-13C]lactate by bacteria isolated from sheep rumen and pig large intestine was compared by the nuclear magnetic resonance (NMR) technique. Washed cell suspensions were incubated directly in the NMR spectrometer and spectra were recorded every 10 min after injection of the labelled substrates. The results showed large differences in the fermentation patterns between rumen and hindgut bacteria. The latter pattern indicated a greater ability for formation and fermentation of lactate than that of the rumen. Moreover, with both substrates the amount of propionate formed via the acrylate pathway was always greater with hindgut than with rumen bacteria, 50% and 20% of the total, respectively.