It is established that feeding heifers a high plane of nutrition post-weaning but before puberty negatively affects mammary gland (MG) development and milk yield. The mass of the MG parenchyma (PAR) increases up to 60-fold from birth until 3 mo of age. Interestingly, enhanced nutrition during this time (pre-weaning) does not appear to negatively affect MG development and may in fact be beneficial. Therefore, the objectives of this study were to clarify the effects of feeding a higher plane of nutrition pre-weaning to Holstein heifer calves on MG growth, and particularly PAR and fat pad (MFP) development. Thirty-six Holstein heifer calves (<1 wk old) were reared on 1 of 2 dietary treatments to create 2 physiologically diverse groups of animals. Diets included (1) a control milk replacer (MR) fed at 454 g powder per day (CON; 20% crude protein [CP], 20% fat), or (2) an accelerated MR fed at 1,135 g powder per day (ACC; 28% CP, 25% fat). Milk replacer mixed at 15% solids was fed as indicated until the 8th week when feeding was reduced 50% to induce weaning. Calves were housed individually with ad libitum access to water. Starter feeding was initiated at wk 5 and balanced between treatments. Udders were examined visually and by palpation with teats measured weekly. At weaning, a subset of calves were killed (n = 6/diet). Whole MG were removed, dissected, and weighed. ACC calves consumed more MR liquid pre-weaning (4575 vs 3199 mL MR/day; P < 0.01). At wk 8, ACC calves had longer front (1.3 vs 0.9 cm; P < 0.01) and rear (1.2 vs 0.8 cm; P < 0.01) teats. Whole untrimmed udders of ACC-fed calves were heavier (255 vs 66 g; P < 0.01). Differences were more pronounced after skin was trimmed (198 vs 38 g; P < 0.01). Dissected PAR and MFP were both greater for ACC calves (10.5 vs 1.4 g and 173 vs 29 g, respectively; P < 0.01). Overall differences remained if expressed on a body weight basis (P ≤ 0.05). Results provide compelling evidence that pre-weaning nutrition markedly affects MG development in dairy calves. Efforts in our lab are underway to uncover the cellular and molecular mechanisms responsible for these differences.