Rabbits were exposed to chlorides of cadmium, copper, or cobalt, for 4-6 weeks (5 days/week and 6 hr/day) at levels ranging from 0.4 to 0.6 mg metal/m3. After exposure to Cd2+ the lungs were enlarged and an interstitial infiltration of neutrophils and lymphocytes as well as intraalveolar accumulations of large, vacuolated macrophages were observed. Morphometrical measurement of volume density of type II cells showed a 2.5-fold increase due to enhancement of cell size as well as cell number. The phospholipid content of lung tissue, determined in the lower left lobe, increased by 40%, mainly due to elevated levels of disaturated phosphatidylcholines. The results indicate that Cd2+ induces a reaction pattern similar to that seen following exposure to Ni2+, in addition to which Cd2+ causes interstitial alveolitis. Exposure to Cu2+ and Co2+ only affected the type II cells. Exposure to Cu2+ resulted in a 1.5-fold increase in volume density, probably due to enhanced number of cells. Following exposure to Co2+ the type II cells formed nodules protruding into the alveolar lumen. However, no significant increase in volume density occurred. The possible association between this abnormal growth pattern and early tumor formation deserves further investigation.