The use of feed additives for ruminants is a further tool that can improve feed conversion in animal protein. The objective of this study was to determine the effect of narasin in mineral mix (MM) of heifers fed high-forage diet. Thirty Nellore heifers (12 mo old) were penned (3 heifers / pen) by BW (222 ± 3.0). Heifers were fed daily with Tifton 85 haylage ad libitum. Experimental treatments were 0 (control); 650 and 1300 mg of narasin / kg MM (0N, 6.5N and 13N, respectively). The variables analyzed were dry matter intake (DMI), and mineral mix intake (MMI) daily, average daily gain (ADG) and feed:gain ratio. The experimental design was a change-over (all pens underwent all treatments) with 3 experimental periods of 28 d. Between periods there was a 7 d wash-out. Heifers were weighed at the beginning and end of each experimental period to determine ADG. The variables ADG and feed:gain were analyzed by PROC MIXED (SAS 9.3), MMI and DMI measures were analyzed as repeated measures over time. Treatments, periods and pens were placed in the model. The addition of narasin in MM did not effected MMI (0N - 60.70 ± 3.05; 6.5N - 55.62 ± 3.18; 13N - 57.44 ± 2.50 g / heifer / d, P = 0.7497), corresponding to 0; 7.30 and 15.62 ppm of narasin intake. There was no change on DMI (0N - 4.85 ± 0.09; 6.5N - 4.95 ± 0.11; 13N - 4.78 ± 0.09 kg / heifer / d, P = 0.6944). However, adding 1300 mg of narasin / kg MM increased ADG (0N - 0.486 ± 0.02; 6.5N - 0.512 ± 0.02; 13N - 0.582 ± 0.02 kg / d, P = 0.0181) and improved feed:gain ration (0N - 12.71 ± 0.85; 6.5N - 11.70 ± 0.85; 13N - 9.50 ± 0.85, P = 0.0488) compared with the control group. In conclusion, adding 1300 mg of narasin / kg MM increased ADG and improved the feed:gain ratio.