Six rumen-fistulated lactating Holstein cows were subjected to temperaturehumidity (T-H) treatments of 18·3°C at 50 per cent relative humidity (RH) and 29·4°C at 85 per cent (RH) in the climatic laboratory. Two feeding treatments, high roughage-limited grain and limited roughage-high grain, were fed. Feed intake was kept constant by feeding any refused feed through the cannula. Rumen fluid was sampled under controlled conditions 2·25 hours post feeding and analysed by chromatography for volatile fatty acids (VFA). Milk production records were kept for all cow and milk was sampled regularly and analysed for fat, protein, total solids and fatty acids. The mean values for the ratios of ruminal acetic acid/propionic acid molar per cent were 2·57: 1 and 3·39: 1 for the high grain and high roughage rations respectively. These were significantly different (P<0·05). There were no significant differences (P> 0·05) between the T-H treatment means in any of the rumen VFA levels. Treatments caused no major changes in milk fatty acid components. There was a significant (P<0·05) T-H x ration interaction effect on milk fat percentage. The cows fed high grain at high T-H had the lowest milk fat (3·42 per cent). T-H treatment had little effect on the milk fat percentage of the cows fed high roughage. These data indicate that high environmental T-H accentuates ration effect on percentage milk fat, and that this response is due to some factor other than VFA levels in the rumen. High ambient T-H appears to affect rumen VFA differently to high grain levels but the mechanism of milk fat depression may be similar in both cases.