[en] Volume regression and regrowth characteristics of rat rhabdomyosarcoma tumors were monitored folowing exposure to plateau (high-energy) and peak (low-energy) charged-particle beams accelerated at the Berkeley BEVALAC facility. Values of the relative biological effectiveness (RBE) for radiation-induced growth delay were obtained for plateau and extended-peak radiation from carbon, neon and argon ions. The RBE values for tumor irradiation in the distal position of a 4 cm peak region of these three ion beams were similar; they ranged from 2.3 to 2.6 for the endpoint of a 50-day delay in growth to twice the volume measured on the day of irradiation. In contrast, RBE values for plateau carbon, neon and argon ions were 1.3, 1.8 and 2.9, respectively, for the 50-day growth delay end point. A therapeutically advantageous peak to plateau RBE ratio was thus obtained only with the carbon-ion and neon-ion beams. These in vivo results are consistent with in vitro measurements of RBE indifferent regions of the Bragg curves of the charged-particle beams used in the present study