Reconstructions from serial ultrathin sections of microvascular endothelium suggest that gaps, which are induced by a range of stimuli, may pass through endothelial cells as well as between them. To address the possibility that the transcellular gaps are not artefacts of aldehyde fixation, we have reconstructed fourteen gaps, induced by the ionophore A23187, in frog mesenteric microvessels where the primary fixative was osmium tetroxide. All fourteen gaps were transcellular. The different actions of osmium tetroxide and glutaraldehyde lead us to consider that it is highly unlikely that transcellular gaps are fixation artefacts.