Pulmonary alveolar macrophages (PAM) from rats exposed to a single dose of ZnO or NiO showed distinct changes in size and ultrastructure. ZnO exposure brought about considerable reduction in the size of the cells, which can be distinguished into two types of macrophages. The macrophages contained a prominent nucleolus, several primary and secondary lysosomes, membrane formations, electron dense structures, and small dense mitochondria. In addition, electron dense and membrane bound structures were seen in the interstitial macrophages and in the intercellular spaces. NiO exposure induced hypertrophy and hyperplasia of the alveolar walls. The macrophages contained invaginated bizarre-shaped nuclei, pigment aggregations, and membrane bound structures. The results seem to suggest that after a one week recovery period following injections with ZnO and NiO, ZnO-related accumulations were transferred from alveolar to interstitial macrophages, however NiO-associated structures were contained within the alveolar macrophages.