The mechanism of estrogen's action on bone mineralization in children has received little attention. Our objective was to determine the effect of time (developmentally) and duration of exposure to an estrogen agonist (zeranol) on bone growth and mineralization using a castrated male lamb model. At birth, 40 male lambs were castrated and within 14 days of birth (day = 0) they were assigned (n = 10 per group) to age-matched control lambs (C-AGE) or to receive a 12.5-mg zeranol implant as follows: E-0, implanted on days 0, 45, 90, and 135; E-90, implanted on days 90 and 135; and E-0,90, implanted on days 0, 90, and 135. Lambs were studied for 163 days. Serum was collected on days 28, 73, 118, 135, and 163 and analyzed for minerals (Ca, P, and Mg), markers of bone remodeling (bone alkaline phosphatase [ALP] and tartrate resistant acid phosphatase [TRAP]), 1,25-dihydroxyvitamin D [1,25(OH)2D], growth hormone (GH), and insulin-like growth factor I (IGF-I). Whole-body bone mineral content (BMC), bone mineral density (BMD), fat mass, and lean mass were determined by dual energy X-ray absorptiometry (DEXA) on days 28, 73, 118, and 163. There was a linear increase in growth at all time points. Whole-body BMC, weight, and lean mass of C-AGE and E-90 lambs were less than E-0, and E-0,90 lambs at all time points. Whole-body BMD of C-AGE and E-90 lambs was less than E-0 and E-0,90 lambs at 28 days and 73 days; however, after implantation at day 90 whole-body BMD of E-90 lambs was similar to E-0 and E-0,90 lambs at day 118 and day 163 and all three were greater than C-AGE lambs. There was no effect of treatment on calcium absorption, serum minerals, hormones, or markers of bone remodeling. We conclude from these data that treatment of growing castrated lambs with an estrogen agonist from birth augments growth, whereas delaying estrogen agonist treatment does not facilitate growth but appears to augment bone mineral accretion. We suggest these observations may have clinical relevance, and deserve consideration when treating children with delays in growth and bone mineral accretion.