Technical Abstract: Quantitative histology of mammary parenchymal tissue from 16 Hereford and 16 Holstein heifers was determined for tissue obtained on day 150, 180, and 280 of gestation and on day 49 of lactation. Percent area occupied by stromal tissue was progressively decreased on each consecutive sample day during gestation in Herefords and was lower in both breeds during lactation. Overall, area occupied by stromal tissue elements was also greater in Herefords. Percent lumenal space and number of alveolar cells per cross section was consistently greater for Holsteins and increased across sample periods in both breeds. More than 40% of the alveolar cells in Herefords were characterized as poorly differentiated but in Holstein heifers nearly all of the cells were classified as either intermediate or fully differentiated. These data provide additional evidence to support the idea that much of the difference in milk production between beef and dairy animals depends on increased mammary function. Whether similar effects occur when Holsteins selected for milk production are compared with Holsteins that less intensively selected for milk production remains to be determined.