Technical Abstract: Our objective was to determine effects of rate of gain and body weight (BW) on mammary parenchymal tissue development. Mammary tissue samples were available from heifers (n = 54) reared on one of two dietary treatments, restricted (R) 650 g/d or elevated (E) 950 g/d of daily gain), and slaughtered at 150, 200, 250, 300, or 350 kg BW. At slaughter, mammary parenchymal samples were excised, preserved, prepared for histology, and stained with hematoxylin and eosin. Representative digital images of tissue sections were captured for analysis. Tissue areas occupied by interlobular stroma, epithelium, lumen, and intralobular stroma were measured (mm2) and the numbers of epithelial and luminal structures per image were tabulated. Mean percentages of mammary parenchyma occupied by interlobular stroma, epithelium, lumen, and intralobular stroma were 28, 20, 7, and 45%, respectively; percentages did not differ by BW or treatment, nor was there an interaction between treatment and BW. Number of epithelial (31 ± 6 vs. 47 ± 5) and luminal (24 ± 5 vs. 38 ± 4) structures per image increased between 150 and 350 kg BW. For heifers slaughtered between 150 and 350 kg of BW, alterations in the rate of gain between 650 and 950 g/d, accomplished by feeding varying amounts of the same diet, had no significant effect on tissue characteristics or the pattern of mammary parenchymal development. Perhaps with other diets, changes in parenchymal tissue composition would be noted. Our findings support the hypothesis that the length of time between birth and slaughter plays a predominant role in determining the tissue characteristics of heifer mammary parenchyma, irrespective of rate of gain.