The aim of the present study was to determine the effects of age and diet (breast milk, formula milk and weaning diet) on metabolic activities in faecal samples from infants aged 1 week to 1 year, and to compare these findings with activities found in samples from adults. Such activities can provide valuable information on functional changes in the microbiota that may have significance for the health of the host. Fresh faecal samples were collected from forty-four breast-fed infants (twenty-four males, twenty females) and thirteen formula-fed infants (three males, ten females) throughout the first year of life. The samples were analysed for protein-breakdown products, including the faecal concentrations of NH3, phenol and p-cresol, and faecal bacterial enzyme activities. There was wide individual variation in all variables measured; however, the values in infants were substantially lower then those found in adults. In pre-weaned infants, faecal NH3 concentration and β-glucuronidase activity were the only endpoints that were significantly different in breast-fed and formula-fed infants (P<0·001 and P<0·05 respectively). This was not apparent after weaning. There was a significant difference between the breast-fed and formula-fed weaned groups and their pre-weaned counterparts only for NH3 (P<0·05). β-Glucuronidase activity and phenol concentration were significantly (P<0·01) greater in weaned breast-fed infants compared with pre-weaned breast-fed infants. No differences were observed between pre-weaned and weaned formula-fed infants for any of the variables except for NH3 concentration. It can be concluded from the present study that there are significant differences in two faecal characteristics between breast- and formula-fed infants and that changes occur as the infants grow older and are weaned onto solid foods.