Application of ultrasound enhances transdermal transport of drugs (sonophoresis). The enhancement may result from enhanced diffusion due to ultrasound-induced skin alteration and/or from forced convection. To understand the relative roles played by these two mechanisms in low-frequency sonophoresis (LFS, 20 kHz), a theory describing the transdermal transport of hydrophilic permeants in both the absence and the presence of ultrasound was developed using fundamental equations of membrane transport, hindered-transport theory, and electrochemistry principles. With mannitol as the model permeant, the role of convection in LFS was evaluated experimentally with two commonly used in vitro skin models- human cadaver heat-stripped skin (HSS) and pig full-thickness skin (FTS). Our results suggest that convection plays an important role during LFS of HSS, whereas its effect is negligible when FTS is utilized. The theory developed was utilized to characterize the transport pathways of hydrophilic permeants during both passive diffusion and LFS with mannitol and sucrose as two probe molecules. Our results show that the porous pathway theory can adequately describe the transdermal transport of hydrophilic permeants in both the presence and the absence of ultrasound. Ultrasound alters the skin porous pathways by two mechanisms: (1) enlarging the skin effective pore radii, or (2) creating more pores and/or making the pores less tortuous. During passive diffusion, both HSS and FTS exhibit the same skin effective pore radii (r = 28 +/- 13 A). In contrast, during LFS, r within HSS is greatly enlarged (r > 125 A), whereas r within FTS does not change significantly (23 +/- 10 A). The observed different roles of convection during LFS across HSS and FTS can be attributed to the different degrees of structural alteration that these two types of skin undergo during LFS.