Expression of the Ha-ras oncogene has been reported to stimulate the dimethylamiloride sensitive Na+/H+ exchanger and Na+,K+,2Cl- cotransport, both transport systems which are involved in cell volume regulation. The present study has been performed to test for an influence of ras oncogene expression on cell volume regulation in NIH 3T3 fibroblasts expressing the Ha-ras oncogene (+ ras). As controls served NIH 3T3 fibroblasts not expressing the ras oncogene (-ras). In isotonic extracellular fluid, the cell volume of + ras cells (2.70 +/- 0.08 pl) is significantly greater than the cell volume of -ras cells (2.04 +/- 0.10 pl). Both, + ras and -ras cells exhibit a regulatory cell volume increase in hypertonic extracellular fluid and a regulatory cell volume decrease in hypotonic extracellular fluid. The regulatory cell volume decrease is inhibited by 1 mmol/l quinidine and barium, the regulatory cell volume increase is inhibited in -ras and +ras cells by dimethyl-amiloride (100 mumol/l) and, only in +ras cells, by furosemide (100 mumol/l) and bumetanide (10 mumol/l). In conclusion, expression of the ras oncogene leads to a shift of the set point for cell volume regulation to greater cell volumes, which may contribute to the activation of the Na+/H+ exchanger and Na+,K+,2Cl- cotransport.