A sampling and measuring device was set up to measure continuously the pH and the redox potential (Eh) of ruminal content in absence of any gaseous contamination (method 1). It was compared with a conventional suction device in which no precaution was taken to prevent air from coming into contact with the surface of collected samples (method 2). Two fistulated dry cows were used and fed a total mixed ration. Redox potential and pH measurements were performed repeatedly on collected samples using these 2 methods during a 9-h period; each period started 1 h before feeding. The partial pressure of oxygen (log fO2) was calculated from Nernst’s equation using pH and Eh values. Results indicated that pH, Eh, and log f(O2) were affected by sampling method. In method 1, pH values ranged from 6.7 to 6.37 and Eh from −173.5 to −216.8 mV. In method 2, pH and Eh values varied, respectively, from 6.93 to 6.49 and from −111.3 to −139.5 mV. The partial pressure of oxygen was 106 times lower in samples that were continuously collected than in hand-samples. As a result, method 1 could make accurate measurements of pH and Eh of ruminal content.