The effect of including monensin sodium at a dose rate of 40 mg/kg in the diet of Friesian bulls (mean initial liveweight 158 kg) fed a barley-based diet ad libitum was assessed in terms of growth rate, feed intake, feed and energy conversion efficiency and the concentration and molar proportions of ruminal volatile fatty acids over a 157-day period. Mean daily feed dry matter intake for the treated group (5.4 kg) was 24 per cent lower than the control group (7.1 kg). The treated group grew at a significantly slower rate than the control animals although feed conversion efficiency was improved by the treatment. A comparison with the results of other workers suggested that the dose rate used had been too high. There were no significant differences in the molar proportions of volatile fatty acids in rumen fluid as a result of monensin usage. While many workers have shown increases in the molar proportions of propionic acid the fact that the control group in this experiment had a very high value (48 per cent) may explain the lack of response.