The use of tetrazolium salts for metabolic studies has been dismissed on the basis of their poor penetration into fresh tissue slices. In view of the fact that the penetration of these compounds can be visualized, it was felt that knowledge of the factors involved would be important. Factors, known to influence the penetration of oxygen, were examined with respect to the tetrazolium salts. The penetration of tetrazolium salt into tissue slices was a regular and predictable phenomenon. It was found that decreasing the metabolism of the cells in the slice substantially increased the penetration of these compounds, while increases in metabolism, by addition of substrate (such as succinate) to the incubating medium, considerably decreased their penetration. Increasing concentrations of the salt in the medium resulted in greater but limited penetration. It is our belief that the metabolism of tissue slices can be effectively studied with the aid of the tetrazolium salts, the portion of the population of cells participating in any reaction being accurately established by measuring the depth of the zone of reduced dye.

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