Rectal administration is a suitable route of administration for drugs that are either very irritating to the intestine (e.g., indomethacin) or are more effective when the first-pass effect in the liver is circumvented. Microemulsions are a tool for the improvement of penetration of sparingly soluble drugs. They are mainly used in topical and transdermal drug delivery. However, they find application also in other routes of administration, mainly due to their ability to solubilize sparingly soluble drugs. The selection of a suppository base depends on the physical properties of the drug. The study focused on evaluating the effect of the microemulsion as the solubilizer of sparingly soluble indomethacin in hydrophilic and lipophilic suppository bases compared with Polysorbate 80 as the excipient contained in the microemulsion. The reference suppositories were prepared by the traditional moulding technique from