Protein content per cell and cell volume of the canine kidney epithelial cell line, MDCK, and one of its chemically induced tumorigenic transformants (T1k) were examined during growth in serum-containing and serum-free, hormonally defined media. Both protein content per cell and cell volume (measured with 3-O-methyl-D-glucose) decreased when cell density increased. Significant inverse linear correlations were found between protein content (and cell volume) and the log of the cell number. Equations relating these variables were derived. No difference was detected between cells grown in serum-containing and in serum-free media. Results of 22Na+ uptake experiments differed markedly when expressed as a function of cell number or as a function of protein content or cell volume. However, protein content and cell volume bore a constant relationship to one another, and expression of results as a function of protein content or cell volume yielded qualitatively similar results. These findings show that in MDCK cells, as in other cell lines, protein content and cell volume decrease markedly during growth and that these variations must be taken into account when expressing and interpreting the results of transport measurements.