Poor penetration of diclofenac through in vitro rat dorsal skin including subcutaneous tissue was observed. The poor penetration of diclofenac seemed to be predominIntly due to the poor permeability of the stratum corneum. Hydrogenated soya phospholipids (phospholipid) in aqueous gel form increased the penetration of diclofenac in the in vitro study, by increasing diclofenac transport through the stratum corneum. In the in vivo percutaneous absorption of diclofenac, the presence of phospholipid in aqueous gel form increased both plasma diclofenac concentration and diclofenac accumulation in the dorsal skin tissue, including subcutaneous tissue. Since a marked accumulation of diclofenac in the subcutaneous tissue after application of the aqueous gel was observed both in vivo and in vitro, percutaneous application of diclofenac in the aqueous gel form, prepared with phospholipid, may be available for topical treatment rather than for systemic treatment.