Gastrointestinal function was assessed in normal foals before, during and after the onset of 'foal heat diarrhoea' by haematological, biochemical, microbiological and parasitological methods, and by a combined lactose tolerance-breath hydrogen excretion test. Warmblood mares (n=11) and their foals were used. The foals were assessed according to a standard protocol after birth and at regular intervals thereafter. All except one of the foals suffered from diarrhoea that started between day 8 and day 12 after birth, which lasted for 1-4 days. Rotavirus was detected in the faeces of five foals, but there was no association with the occurrence of diarrhoea. A significant increase in aldosterone concentration compared with that at days 6 and 18 was observed in five foals on the second day of diarrhoea. There were no differences between age groups in plasma glucose concentrations after lactose administration. Peak breath hydrogen excretion after lactose administration ranged from 17 to 232 p.p.m., and increased with age of foal. There was no decrease in lactose absorption or increase in breath hydrogen excretion in foals suffering from diarrhoea.