The effect of monensin on the in vitro degradation of cellulose was investigated using ruminal fluid obtained from sheep adapted and unadapted to the antibiotic. In vivo ruminal adaptation to monensin was also examined over a six-week treatment period. Results showed that monensin severely inhibited total VFA production when cellulose was offered as substrate to unadapted ruminal organisms. There was only a small decrease when the rumen inoculum was obtained from sheep fed monensin for 32 days. Addition of monensin to the diet resulted in higher propionate and lower butyrate concentrations when compared with values obtained from sheep fed diets without monensin. The addition of monensin decreased dilution rate and increased rumen volume.