Previous studies in human foreskin fibroblasts suggested that the mechanism by which serum stimulates Na+ influx is via a Ca2+-calmodulin-mediated event. In the present experiments in normal WI-38 cells (human lung fibroblasts), both the intracellular Ca2+ antagonist 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8) and the potent calmodulin antagonist trifluoperazine (TFP) blocked serum-stimulated Na+ influx [TMB-8 concentration causing half-maximal inhibition (Ki) = 15 microM and TFP Ki = 10 microM]. Similar results were obtained in Swiss 3T3 cells. In contrast, in transformed WI-38 or Swiss 3T3 cells neither TMB-8 nor TFP had any effect on serum-stimulated Na+ influx (TMB-8 Ki greater than 100 microM and TFP Ki greater than 100 microM). In addition, when 45Ca2+ efflux measurements were made on normal and transformed cells, serum stimulated significant 45Ca2+ efflux (P less than 0.05) from WI-38 and Swiss 3T3 cells, while having no effect on 45Ca2+ efflux from simian virus 40 (SV40)-WI-38 or SV40-Swiss 3T3 cells. However, an elevation of intracellular Ca2+ can stimulate Na+ influx, since it was found that A23187 mimicked the effects of serum in both normal and transformed cells. These results suggest that the Ca2+-calmodulin-mediated event, which is thought to be involved in serum-stimulated Na+ influx in normal cells, may be bypassed or overridden in transformed cells.