High-production dairy and beef systems require diets rich in starch. This practice may induce ruminal acido- sis and also increase exposure to mycotoxins because starches in starch-rich diets are the main vehicles of mycotoxin contamination. The aim of this study was to investigate the effects of low ruminal pH on the bioavailability of 4 major mycotoxins [i.e., aflatoxin B 1 (AFB1), ochratoxin A (OTA), deoxynivalenol (DON), and fumonisin B 1 (FB1)]. Eight nonlactating dairy cows fitted with rumen cannulas were used in a double crossover experiment. The trial was divided into 4 pe- riods with 2 periods per crossover. Cows were divided into 2 groups receiving a low (15% dry matter basis) and high-starch diet (30.8%) with and without live yeast supplementation (1 × 10 10 cfu per cow) in the first and second crossover, respectively. At the end of each period, cows received a single dose of mycotoxin- contaminated feed containing 0.05, 0.2, 0.24, and 0.56 mg of AFB1, OTA, DON, and FB1 per kg of feed, respectively. The fecal and urinary excretion of myco- toxins and their metabolites was monitored for up to 48 h postdosing. As expected, ruminal pH decreased in cows fed the high-starch diet. The high-starch diet in- creased the bioavailability of OTA and AFB1. Urinary excretion of OTA 24 h after mycotoxin administration increased 3-fold in the high-starch diet, correlated with lower fecal excretion. Similarly, a decrease in fecal ex- cretion of AFB1 was accompanied by an increase in urinary excretion of its major metabolite, aflatoxin M 1 , 48 h after mycotoxin administration. In contrast to AFB1 and OTA, the bioavailability of DON and FB1 remained unchanged. Yeast supplementation had no effect on the excretion balance of these 2 mycotoxins. In conclusion, these results show that high-starch diets increased the bioavailability of OTA and AFB1, most probably through the lowering effect on ruminal pH. This greater bioavailability potentially increases the toxic effects of these mycotoxins.