Intramammary infections (IMI) are prevalent in lactating and nonlactating dairy cattle and their occurrence during pregnancy is concerning given the considerable amounts of mammary growth and development that occur during late gestation, in preparation for lactation. To date, the effects of IMI on mammary growth and development are largely unknown. This study’s objectives were to determine how IMI affected total and differential mammary secretion somatic cell counts and characterize mammary morphology changes resulting from IMI in glands stimulated to grow with estradiol and progesterone. Mammary growth was induced in 19 nonpregnant, nonlactating dairy cows and 2 quarters free of IMI of each cow were subsequently infused with either saline (n = 19) or Staphylococcus aureus (n = 19). Mammary secretions were collected throughout the trial and mammary tissues were collected at either 5 or 10 d post-challenge. Secretion and tissue data were analyzed using a mixed model with treatment and euthanasia day as fixed effects and cow nested within euthanasia day as random. Infected quarter secretions contained a greater concentration of somatic cells than saline infused quarters (7.45 vs 6.77 ± 0.06 log₁₀ cells/mL; P < 0.001), and contained a greater proportion of neutrophils than saline quarter secretions (47.2% vs 7.1% ± 2.3%; P < 0.001). Infected mammary tissues also exhibited greater degrees of immune cell infiltration than saline quarters. Lobules in Staph. aureus mammary tissues displayed a greater percentage of luminal space (7.7% vs 5.4% ± 0.6%; P = 0.004), a reduced percentage of epithelial area (33.3% vs 38.1% ± 1.1%; P < 0.0001), and tended to have a greater percentage of intralobular stromal area (59.0% vs 56.5% ± 1.3%; P = 0.1) than saline quarters. These results indicate that IMI in nonlactating glands that were stimulated to grow, elicited an immune response leading to an infiltration of immune cells into mammary tissues and secretions, which was associated with changes in mammary tissue structure. The observed reduction of epithelial areas suggest that IMI negatively affects mammary growth, which may result in reduced future milk yields and productive herd life.