Recent electron probe analytic studies of freeze-dried cryosections of vascular smooth and vertebrate striated muscle are reviewed. The results show that the sarcoplasmic reticulum of striated muscle is not in ionic communication with the extracellular space. Vacuolation by hypertonic solutions and fatigue involves the T-tubule system. The high calcium content of the terminal cisternae of the resting muscle has been quantitated in situ. In smooth muscle, the high Cl content is distributed in the cytoplasm, and mitochondria in rabbit portal vein smooth muscle cells do not contain high concentrations of calcium. Mitochondrial calcium loading in the form of granules is generally due to fiber damage. Nuclear and mitochondrial composition in situ has been quantitated and compared to the composition of the cytoplasm of the same cells. Preliminary phosphorus x-ray maps of smooth muscle show the feasibility of this approach in defining the composition of organelles in thin cryosections. The use of x-ray maps at intermediate resolution is illustrated with tropomyosin paracrystals labelled with Hg-containing dye at the thiol residues. Mercury x-ray maps of such paracrystals show the 40nm periodicity of the thiol groups and their Fourier transforms contain information to a spatial resolution of 10-20nm.