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Ninety-six yearling Hereford heifers (mean = 232 +/- 3 kg) were stratified on the basis of weight and then allotted randomly within weight groups into 16 pens of six each. Treatments were Hei-Gro (deviced or nondeviced; HG), Synovex-H (implanted or nonimplanted) and location in the feedlot relative to steers (isolated or nonisolated) imposed in a 2 x 2 x 2 factorial design with two replicates. During the growing phase, estrous activity was monitored by weekly rectal palpation of the reproductive tract of three heifers from each pen. Incidence of standing estrus, as determined by daily visual observation, was recorded for the first 21 d of the finishing phase. Heifers implanted with Synovex showed higher (p less than .05) average daily gains and final shrunk weights than nonimplanted heifers during the growing phase. Synovex also resulted in increased (P less than .05) weight gain during the finishing phase and produced heavier (P less than .05) hot carcass weights. HG did not significantly (p less than .05) affect any performance traits reported. Synovex or HG treatments had no effect on estrous activity of the heifers. Location significantly (P = .05) affected estrous activity during the finishing phase. HG caused reproductive tract scarring and infection (P less than .01). Heifers treated with the HG + Synovex combination had more severe (P less than .05) infection than those treated with HG alone. Heifers not isolated from steers had larger (P less than .05) ovaries than isolated heifers, although no difference (P less than .05) was found in ovarian weight.