In the first 3 weeks of age the calf is most susceptible to diarrhoea. In a review of the literature, Williams (1977) concluded that the lower critical temperature of 3-day-old calves on restricted levels of feeding was between 9°and 13°C and by approximately one month of age was between 0° and 5°C. these temperatures can be increased by air movement (Blaxter and Wainmann, 1964). Grimble (1982) suggested that calves should not be exposed to air speeds greater than 0.25 m/s. Studies in older cattle and sheep, with a functional rumen, have shown that exposure to cold was associated with a lower apparent digestibility of feed, irrespective of changes in feed intake (Christopherson, 1976). Williams and Innes (1982) fed a milk substitute diet to calves from 24 to 38 days of age and compared housing the calves in air temperatures of 3° and 18°C. There were no significant differences between air temperatures in the apparent digestibilities of dry matter (DM), fat and nitrogen (N) (p>0.05) but there were significant increases in faecal water excretion and significant decreases in water intakes and urine volumes at 3° compared to 18°C (p<0.05). The present experiment was designed to determine the effects, at different feeding levels, of continuous exposure to cold (10°C) compared with warm (25°C) air and to low (<0.2 m/s) compared with high (>3.0 m/s) air speeds on (a) the apparent digestibility of a high fat milk substitute diet, (b) water intakes and faecal water excretions and (c) growth rates, in calves from 2 to 24 days of age.