Abstract: ABSTRACT: OBJECTIVE To assess the transdermal and in vivo biotransformation differences between two kinds of formulations of topical applied aminolevulinic acid hydrochloride (ALA) in the form of solution and gel. METHODS Transdermal behevier of the two formulations were performed in Franz diffusion cell; ALA converted protoporphyrin(PpIX) in mice skin was also quantified as an index to assess transformation efficiency of the two formula. RESULTS Cumulative retained and permeated(Q) amount of ALA in pig skin, transdermal flux and lag time(TL) of ALA gel was 59.90 ? 28.68 ?g?cm-2, 2135.8 ? 223.5 ng?mg-1, 12.27 ? 5.99 mg?cm-2?h-1, and 0.70 ? 0.35 h; The same results got from ALA solution was 52.68 ? 9.95 ?g?cm2, 2173.5 ? 480.8 ng?mg-1, 13.22 ? 5.82 mg?cm-2?h-1and 0.80 ? 0.32 h; Biotransformed PpIX from ALA was 41.91 ? 18.80 ?g?cm-2 (for gel) and 38.83 ? 15.70 g?cm-2(for solution), respectively. CONCLUSION There are no statistically differences between ALA gel and solution both in transdermal behevier and PpIX transformation efficiency, based on in vitro and vivo experiments.