We have found that hypergravity stimulates the proliferation of HeLa cells through reduction of the G1 phase duration, concomitant with enhancement of c-myc gene expression. HeLa cells were grown in monolayer in culture flasks that were centrifuged to generate a constant 18, 35 or 70 g at 37 degrees C for up to 4 days. The cell proliferation was enhanced at 18, 35 and 70 g, most notably at 35 g. Cell cycle analyses with [3H]thymidine (TdR)-colcemid treatment showed that the cell generation time in the 35 g culture was reduced by 17% as compared to the control, which was attributed to a 26% reduction of the G1 phase duration. No differences were observed in the duration of the S, G2 and M phases or in the [3H]TdR incorporation per S phase cell between the 35 g culture and the control. The induction of c-myc gene expression was investigated by RNA blot hybridization during a 15-360 min exposure of cells to 18, 35 and 70 g. Elevated levels of c-myc mRNA were observed after a 15-min exposure, and maintained after a 360-min exposure at all hypergravities examined. The highest induction rate of c-myc mRNA was 3.8-fold higher than the control after a 120-min exposure to 35 g. The 35 g condition was the most effective hypergravity for stimulating both cell proliferation and c-myc gene expression. Our study suggests that the appropriate level of hypergravity stimulates HeLa cell proliferation by reducing the G1 phase duration without affecting DNA synthesis rate, mediated through induction of c-myc gene expression.