Synergistic effects of sonophoresis and iontophoresis oil skin penetration were investigated using vitamin B-12 (VB12, molecular weight 1355.4 and non-ionized compound at pH 7.4) as a model drug in the stratum corneum (SC) of hairless mice. Under passive penetration, SC acts as an effective diffusion barrier for VB12 and physical treatment is necessary to enhance skin penetration flux. Ultrasound (US) treatment (frequency of 300 kHz. intensity of 5.21 W/cm(2), and pulse mode of 5.4% duty cycle) under sonophoresis increased both VB12, solubility and diffusivity. in the skin according to its energy flux [J/cm(2)] (intensity x treatment time x duty cycle). The penetration flux of VIS, treated with US of 510 J/cm(2) was 12 times larger than that through intact skin. Iontophoresis (IP) application increased the convective water flow due 10 electro-osmosis, and resulted in in increase in the flux of non-electrolyte VB12 with IP (0.3 mA/cm(2)) by 20 times compared to the flux of passive experiments. Using US and IP combination, a synergistic effect oil the penetration flux of VB12, was achieved. This synergistic effect mail be caused by a different mechanism than the one responsible for enhancing skin penetration with only US or IP. Thus, the combination of US and IP treatment is an effective met hod for skin penetration of large molecules which enter in to systematic circulation with great difficulty.