Our objective was to determine effects of feeding prepubertal heifers (n=16/trt) for rapid gains for different lengths of time (0, 3, 6, 12 wk) on body and mammary growth. Increasing time on the high diet increased mass of carcass and extraparenchymal mammary fat but did not alter mammary parenchyma weight. Increasing time on the high diet decreased mass of mammary parenchyma per kg carcass in a linear fashion. Although mass of parenchyma was not decreased by feeding the high diet for only 3 or 6 wk, changes were consistent with impaired growth seen with 12 wk of the high diet.