The objectives of this study were to evaluate the influence of diet composition on ruminal parameters, more particularly redox potential (Eh). Four Holstein dry dairy cows, fitted with ruminal cannulas, were allocated in a 4 × 4 Latin square design. They were given four experimental hay-based diets D0, D25, D42 and D56 consisting of 0%, 25%, 42% and 56% of ground wheat and barley concentrate mixture, respectively. They were fed at a daily feeding rate of 8.0 kg DM per cow during a 24-day experimental period (a 21-day diet adaptation, three consecutive days of measurement and sampling). The physicochemical parameters, such as pH and Eh, were measured and Clark’s exponent (rH) was calculated from 1 h before feeding to 8 h after feeding at 1-h interval. Samples of ruminal fluid were taken at 0, 1, 2, 4, 6 and 8 h after feeding for the determination of volatile fatty acid (VFA) and ammonia N (NH3-N) concentrations. Ruminal bacterial populations were also studied by means of capillary electrophoresis single-strand conformation polymorphism (CE-SSCP) technique to focus on the structure of the ruminal microbiota and the diversity index was calculated. Mean ruminal Eh and rH were not modified by the concentrate-to-forage ratio and averaged – 210 mV and 6.30, respectively, across diets. The pH decreased slightly by 0.10 pH unit between treatments D0 and D56 with an average of 6.58. Nevertheless, the time during which physicochemical parameters remained at nadir value after feeding varied with diets: 2 and 7 h for D0 and 6 and 5 h for D56, respectively for pH and Eh. Moreover, fermentative parameters were altered by treatments: total VFA and NH3-N were greater in D56 (72.2 mM and 17.5 mg/100 ml) compared with D0 (65.2 mM and 14.2 mg/100 ml). However, neither the structure of bacterial populations of the rumen nor the diversity index (Shannon) was altered by treatments.