Naproxen (Np) is an example of a non-steroidal anti-inflammatory drug (NSAID) commonly used for the reduction of pain and inflammation. In order to develop an alternative formulation for the topical administration of Np, microemulsions were evaluated as delivery vehicles. Four formulations were prepared using isopropyl myristate (IPM) as oil phase, Span 80, Labrafil M, Labrasol, Cremophor EL as surfactants, ethanol as co-surfactant and distilled water or 0.5N NaOH solution as aqueous phase. The final concentration of Np in the microemulsion system was 100 mg/g (w/w). The physicochemical properties such as electrical conductivity, droplet size, viscosity, pH and phase inversion temperature of microemulsions were measured. Stability tests of the formulations were also performed at 5 +/- 2, 25 +/- 2 and 40 +/- 2 degrees C. The abilities of various microemulsions and selected commercial (C) formulation to deliver Np through the skin were evaluated in vitro using diffusion cells fitted with rat skins. The in vitro permeation data showed that microemulsions increased the permeation rate of Np between 4.335-9.040 times over the C formulation. Furthermore Np successfully permeated across the skin from the microemulsion with the highest flux rate (1.347 +/- 0.005 mg . cm(-2) . h(-1)) from a formulation (M4(Np)) consisting of IPM (2.36g), Labrosol (0.13g), Span 80 (0.62g), ethanol (5.23 g), 0.5N NaOH solution (0.66g) and Np (1 g). According to the histological investigations, no obvious skin irritation was observed for the studied microemulsions. These results indicate that the microemulsion formulation may be appropriate vehicles for the topical delivery of Np.