Several studies suggest that aquaporin water channels can be identified in membranes by freeze-fracture electron microscopy. For this report, Chinese Hamster ovary cells were stably transfected with cDNAs encoding aquaporins 1-5. Measurement of the osmotic water permeability of the cells confirmed that functional protein was expressed and delivered to the plasma membrane. By freeze-fracture electron microscopy, a 20% increase in intramembrane particle (IMP) density was found in plasma membranes of cells expressing AQP2, 3 and 5, and a 100% increase was measured in AQP1-expressing cells, when compared to mock-transfected cells. On membranes of cells expressing AQP4, large aggregates of IMPs were organized into orthogonal arrays, which occupied 10-20% of the membrane surface. IMP aggregates were never seen in AQP2-transfected cells. Hexagonally packed IMP clusters were detected in approximately 5% of the membranes from AQP3-expressing cells. Particle size-distribution analysis of rotary shadowed IMPs showed a significant shift from 13. 5 (control cells) to 8.5 nm or less in AQP-expressing cells; size distribution analysis of unidirectionally shadowed IMPs also showed a significant change when compared to control. Some IMPs in AQP expressing cells had features consistent with the idea that aquaporins are assembled as tetramers. The results demonstrate that in transfected CHO cells, AQP transfection modifies the general appearance and number of IMPs on the plasma membrane, and show that only AQP4 assembles into well-defined IMP arrays.