Thirty-two midlactation dairy cows were fed either a typical dairy diet or the same diet plus 340 mg lasalocid/d for 98 d. Diets were 65% forage (alfalfa and corn silage) and 35% concentrate (DM basis). Lasalocid did not affect production of milk (21 kg/d) or FCM (20 kg/d) or milk composition. Dry matter intake was slightly lower for cows consuming lasalocid than for control cows (19.6 vs. 20.6 kg/d). Lasalocid improved energetic efficiency by about 20% during the first 2 wk of the experiment, but treatment effects diminished as the experiment progressed. The period in which lasalocid had significant effects on energetic efficiency was also the period in which lasalocid increased ruminal propionate and decreased ruminal acetate concentrations. On d 7 of the experiment, cows fed lasalocid had lower acetate to propionate ratios as compared with control (3.0:1 vs. 3.7:1). No effect of treatment was observed on ruminal VFA during the remainder of the experiment. These data are interpreted to show that lasalocid improved the efficiency of converting dietary digestible energy into NE1 by altering ruminal fermentation, but this effect was relatively short-lived, since treatment effects on ruminal VFA patterns and energetic efficiency became negligible by 28 d.