Elevated levels of nitrates in feed and water can pose a significant risk for dairy cattle, due to their cumulative action. The effect of prolonged consumption of water naturally contaminated with nitrates on some metabolic parameters in dairy cows was investigated at the present study. Concurrently, whether in-feed inclusion of clinoptilolite, a natural zeolite with high selectivity for ammonia cations, could ameliorate nitrate consumption consequences was examined. Two experiments were run simultaneously in two farms each. In both, farms were assigned into two groups according to nitrate levels in borehole water (NG > 40 ppm; CG < 40 ppm). Furthermore, in experiment 2, the incorporation of clinoptilolite in the ration was taken into account (NC-clinoptilolite feeding; CNC-controls). In experiment 1, blood urea nitrogen (BUN) and beta-hydroxybutyrate (BHBA) concentrations appeared to be affected by nitrate consumption and were significantly higher in NG animals. In experiment 2, BUN concentration was significantly lower in the NC group. The prolonged consumption of water with increased nitrate levels seemed, to some degree, to impair protein metabolism and glucose utilization, while the dietary administration of clinoptilolite could alleviate the nitrates' effects.