Guinea pigs were exposed to cotton dust (25 mg/m³) generated by a Pitt-3 system for 6 hrs daily Monday through Friday and again on the following Monday. Animals were sampled 17 hrs following exposure on days 1, 2, 3, 5, and 8 and their pulmonary responses to inhalation of cotton dust compared to appropriate controls. Breathing rate increased by 70% in animals exposed for 1 day; this enhancement declined throughout the remainder of the exposure week and rose again after exposure on the following Monday to a rate 25% above its control. Lavagable leukocytes increased 11-fold after the first exposure and peaked (14-fold increase) after the second exposure; leukocyte influx declined from this level for the remainder of the exposure week and rose again after exposure on the eighth day to a level 3-fold above its control. Lavagable alveolar macrophages decreased slightly (14%) after the first exposure, increased to 82% above control after the second day, and peaked (154% increase above control) after the fifth day; macrophage yield declined from this peak level by 33% after exposure on the following Monday. Zymosan-stimulated superoxide anion secretion from these alveolar macrophages increased by 40% after the first exposure, declined from this level throughout the remainder of the exposure week, and rose again after exposure on the following Monday to a value of 35% above its control. These data indicate that breathing rate, leukocyte influx, and particle-stimulated superoxide release from macrophages exhibit "Monday accentuation." In contrast, macrophage yield increased with consecutive exposures and did not exhibit "Monday accentuation."