Spectrophotometric method is often recommended in dissolution testing of not only tablets or capsules but also suppositories. Dissolution rate of diclofenac sodium was studied using a pharmacopoeial flow-through apparatus and pH 7.3 buffer. For two lipophilic suppository bases (Witepsol W-32 and H-19) dissolution rate was overestimated with the spectrophotometric zero-order method in comparison to HPLC method, although the difference was statistically significant only in the case of the Witepsol W-32 basis. On the other hand, first derivative spectrophotometry allowed for elimination of the interference of the suppository lipophilic components dissolved in the buffer medium and no difference was observed between the results of this analysis in comparison with HPLC results.