In the HeLa tumor cell line, we studied the characteristics of the dual effect of digitalis compounds on cell growth (proliferation and death). In addition, we explored whether both effects occur by means of the same mechanism. HeLa cell cultures were exposed to increasing concentrations (0.01 nM-10 microM) of ouabain, strophantidin, digoxin, and digoxigenin at 24-96 h intervals. Cell growth in treated cultures was compared with cell growth under nontreated conditions. Additionally, we studied changes in nuclear morphology, as well as in genomic DNA degradation, cytochrome c release, and caspase-9 and -3 presence and processing induced by toxic concentrations of digitalis. Digitalis compounds increased HeLa cell number when exposed to concentrations <10 nM during a 48 h period. Ethacrynic acid (a nonsteroid inhibitor for Na+/K+-ATPase) did not induce cell growth at these concentrations. Digitalis concentrations >10 nM induced cell death in a concentration- and exposure period-dependent fashion. Changes in nuclear morphology, DNA fragmentation, mitochondrial cytochrome c release, and proteolytic processing of caspases-9 and -3, suggest apoptotic cell death. The IC50 for the inducing effect of apoptosis by ouabain at 96 h was 18 nM and corresponds with the IC50 for the Na+/K+-ATPase inhibition in HeLa cells. In conclusion, the dual effect of digitalis compounds on HeLa cells growth is concentration and time-dependent. The apoptosis-inducing effect correlates with inhibition of Na+/K+-ATPase. Proliferation does not appear to be mediated through this pathway. The apoptosis-induction pathway is possibly cytochrome c-dependent.