The effects particle size of corn grain and the ionophore lasalocid on ruminal fermentation and feeding behavior of early lactation cows were examined. Eight multiparous and 4 primiparous cows in early lactation were fed diets (44% forage) with cracked or ground dried shelled corn grain and with or without lasalocid (360 mg/d per cow). The experiment was a replicated (n = 3) 4 x 4 Latin square design with 21-d periods and a 2 x 2 factorial arrangement of treatments. Ground corn decreased ruminal turnover time of starch, did not affect ruminal lactate concentrations, increased propionate, decreased the ratio of acetate to propionate, and decreased branched-chain fatty acids. The range of ruminal pH within a day increased with ground corn, but mean pH was unaffected by treatment. Lasalocid increased lactate concentrations and did not affect the ratio of acetate to propionate. Lasalocid increased total time spent ruminating, and ground corn decreased ruminal contractions. Lasalocid tended to increase water intake, and ground corn increased water intake. Interactions of lasalocid and particle size of the corn grain were observed for ruminal turnover time of starch and NDF, and ruminal pool size of acetate and total VFA.