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The aim of this study was to determine the effects of three additives (Narasin, Lasalocid and Virginiamycin) on rumen fermentation characteristics of steers fed high-forage diet. Thirty-two rumen-fistulated Nellore steers were assigned to a randomized completed block design, according to their initial BW. Steers were fed daily and diets were composed of 99% of coastcross haylage and 1% of concentrate, used as delivery vehicle for the additives. The concentrate and forage were offered separately, whereas the forage was offered after the entire consumption of the concentrate. The experimental diets consisted of: CON = Control; NAR: 13 ppm of narasin; LAS = 20 ppm of lasalocid; and VIR: 20 ppm of virginiamycin. The experimental period lasted 140 d, divided on 5 periods of 28 d each. Rumen fluid was collected on d 0, 28, 56, 84, 112 and 140, at 0, 6 and 12 hours after feeding and combined as a pool for SCFA analysis. Data were analyzed as repeated measures over time using the MIXED procedure of SAS and the LSMEANS option was used to generate individual means. Data obtained on d 0 were used as covariate. Orthogonal contrasts were determined to mean comparisons (C1: CON vs NAR, LAS, VIR; C2: NAR and LAS vs VIR; C3: NAR vs LAS). There was a treatment effect on total SCFA (87.13, 105.42, 88.64 and 90.35 mM/100mM; C1: P = 0.02; C2: 0.06; C3: P < 0.01), propionate (14.47, 15.74, 14,77 and 14,50 mM/100mM; C1: P = 0.04; C2: P = 0.01; C3: P < 0.01), and Ac:Pr ratio (5.19, 4.50, 5.05 and 5.23; C1: P < 0.01; C2: P < 0.01; C3: P < 0.01). Therefore, the inclusion of narasin increased propionate, total SCFA and decreased the Ac:Pr ratio compared to the other feed additives.