The objective was to determine the effect of feeding monensin on growth performance, average age at breeding, and body composition of Holstein heifers. Forty heifers were divided by weight into two groups, averaging 330 kg (heavy) and 217 kg (light) at the beginning of the trial. Within each weight group, heifers were assigned randomly to either of two treatments: without or with monensin feeding (200 mg per head daily). All heifers were fed diets containing either corn silage at 2% BW or ad libitum grass pasture supplemented with concentrates (1 to 1.5 kg per head daily). Monensin had no effect on BW, average daily gain, height at withers, heart girth, length, coccae width, or body condition score. Feeding monensin significantly decreased the age at breeding by 15 and 24 d and age at calving by 36 and 61 d for heavy and light heifers, respectively. Estimated empty body water, protein, or fat percentages determined by urea space were not affected by monensin. Heavy heifers contained a smaller percentage of empty body water but higher percentages of empty body protein and fat than light ones. The results from this study indicate that monensin can reduce age at puberty in Holstein heifers without affecting BW and composition.