Guinea pigs of three different weight ranges (250, 350 and 500 grams) were exposed to 11 mg/m³ respirable cotton dust to determine whether the pulmonary response was influenced by the age (i.e. weight) of the animals. Breathing rates in air and 10% CO2 were measured pre- and post-exposure on all animals. Half of each exposed group and each control group were sacrificed immediately postexposure for measurement of residual gas volume as an index of airway closure. Additional breathing rates were measured in the remaining animals every three hours, up to 18 hours, post-exposure at which time the animals were sacrificed for pulmonary lavage. Total and differential cell counts were made for cells harvested from the lavage fluid and superoxide anion release from alveolar macrophages was measured. Results showed that the group of larger animals was significantly less responsive to cotton dust than the smaller animals.