A method is described whereby the rumen pH in sheep entirely nourished by infusion of volatile fatty acids was determined continuously. The electrodes used were standard laboratory combination electrodes connected to a digital pH meter, a multi-channel chart recorder and an isolation module. The correlation between pH readings from the bench samples and from electrodes immersed in the rumen for periods of up to 28 days was 0.980 +/- 0.015 for 250 observations. There were fairly regular patterns of change in ruminal pH. The pH rose and fell by approximately 0.1 to 0.2 units over a period of about one hour. These changes were thought to be associated with periods of pseudorumination.