This work was originally undertaken to determine the effective conditions of essential oils against Trichophyton mentagrophytes in vitro for the treatment of tinea pedis in a foot bath. Agar blocks implanted with T. mentagrophytes were immersed in 0.1% aqueous agar containing two-fold dilutions of essential oils with or without sodium chloride at 27 degrees C, 37 degrees C and 42 degrees C for 10 and 20 min. The number of surviving mycelia on the agar blocks was determined from the standard curves of the colony diameter and original inocula of the conidia. At the same time, the thermal effect on the cellular morphology was examined using SEM. Most fungal mycelia (99.7%) were killed after treatment at 42 degrees C for 20 min without essential oil. The fungicidal activity of essential oils was markedly enhanced by treating at 42 degrees C for 20 min as compared with that at 27 degrees C, showing 1/4 - 1/32-fold reduction of minimum fungicidal concentration (MFC to kill 99.99%). The order of the fungicidal activity of 11 essential oils was oregano, thyme thymol, cinnamon bark > lemongrass > clove, palmarose, peppermint, lavender > geranium Bourbon, tea tree > thyme geraniol oils. MFCs were further reduced to 1/2 - 1/8 by the addition of 10% sodium chloride. The salt effect was explained, at least partly, by an increase in mycelial adsorption of antifungal constituents in the presence of sodium chloride. Considerable hyphal damage was done at 27 degrees C by the essential oils, but no further alteration in morphology of the hyphae treated at 42 degrees C with or without oil was observed by SEM. The inhibitory effect of heat and oils was also observed against mycelia of T. rubrum and conidia of T. mentagrophytes. Thermotherapy combined with essential oils and salt would be promising to treat tinea pedis in a foot bath.