Seventy-two Simmental bull calves were allotted randomly at birth to either a zeranol (Ralgro) implanted (I) treatment or a nonimplanted control (C) group. At birth, bulls assigned to the I treatment were implanted with 36 mg of zeranol and reimplanted at average intervals of 84 d until slaughter. Calves were weaned at 7.2 mo, preconditioned for 2 wk, and randomly assigned by treatment to slaughter ages of 12.0, 13.8, 15.7 and 17.4 mo and fed an 83% concentrate diet until slaughter. At 7.7 mo and all slaughter ages, live weights and hip heights were similar (P greater than .10), but I bulls had lower (P less than .05) masculinity scores than C bulls. Scrotal circumferences for I bulls were smaller (P less than .05) at 7.7, 12.0, 13.8 and 15.7 mo, but similar (P greater than .10) at 17.4 mo to C bulls. Behavioral observations of passive bunting, mounting attempts, facility rubbing and overall activity were less (P less than .05) for I bulls than C bulls from 12.0 to 13.8 mo but similar (P greater than .10) from 13.8 to 17.4 mo. Aggressive bunting was less (P less than .05) for I bulls during five of the seven observation times for the 12.0-to-13.8-mo period only. Slaughter and carcass weights and dressing percentages were similar (P greater than .10) for I and C bulls. Skeletal maturity and marbling scores, fat thicknesses, yield grades and 9-10-11th rib fat percentages were higher (P less than .05) and lean maturities, forehead hide weights, high dermis thicknesses, femur measurements and 9-10-11th rib bone weights and percentages were lower (P less than .05) for I than C bulls. Rib eye areas and carcass masculinity traits of jump muscle and crest of I bulls were less (P less than .05) at 13.8 mo but reached similar (P greater than .10) development to C bulls at 15.7 mo. Testicle weights of I bulls were lighter (P less than .05) at 12.0, 13.8 and 15.7 mo, but similar (P greater than .10) to C bulls at 17.4 mo. Growth traits (all weights plus hide and femur measurements) and maturation traits (skeletal and lean maturities, marbling scores, fat thicknesses and pizzle eye sizes) increased the most from 12.0 to 13.8 mo and 13.8 to 15.7 mo, respectively. We concluded that implanting young bulls with zeranol from birth to slaughter had minimal effects on live measurements, improved carcass desirability and delayed masculinity and behavioral development.