The capability of 17β-estradiol (E2) to induce the non-genomic activities of its receptors (ERα and ERβ) and to evoke different signaling pathways committed to the regulation of cell proliferation has been analyzed in different cell cancer lines containing transfected (HeLa) or endogenous (HepG2, DLD1) ERα or ERβ. In these cell lines, E2 induced different effects on cell growth/apoptosis in dependence of ER isoforms present. The E2–ERα complex rapidly activated multiple signal transduction pathways (i.e., ERK/MAPK, PI3K/AKT) committed to both cell cycle progression and apoptotic cascade prevention. On the other hand, the E2–ERβ complex induced the rapid and persistent phosphorylation of p38/MAPK which, in turn, was involved in caspase-3 activation and cleavage of poly(ADP-ribose)polymerase, driving cells into the apoptotic cycle. In addition, the E2–ERβ complex did not activate any of the E2–ERα-activated signal molecules involved in cell growth. Taken together, these results demonstrate the ability of ERβ isoform to activate specific signal transduction pathways starting from plasma membrane that may justify the effect of E2 in inducing cell proliferation or apoptosis in cancer cells. In particular this hormone promotes cell survival through ERα non-genomic signaling and cell death through ERβ non-genomic signaling. © 2004 Wiley-Liss, Inc.