We are currently using a BioRad MRC600 system to examine structure and physiology of capillary blood vessels in the frog and hamster mesentery. We have various projects, among which are: 1) examination of the 3D orientation of endothelial cells by observation of very light silver staining of the endothelial cleft and 2) measurement of the temporal and spatial development of solute diffusion gradients in tissue surrounding individually perfused capillaries as well as capillary wall permeability coefficients. (The latter project is being done in the x-z plane with recording over time--so our three dimensions are really x-z-t at present.) These projects are being done on live tissue. We use measurements of permeability before and after laser illumination to check for tissue viability/damage. Other significant points that we have investigated related to the technology concern 1) the apparent elongation in z due to mismatch of refractive indeces and 2) attenuation of fluorescent intensity with depth in tissue. Roger Adamson Dept. Human Physiology School of Medicine Univ. Calif. Davis