There are many drugs reproted to show unusual pharmacokinetic behavior by producing a significant secondary peak in the plasma concentration-time curve after oral administration. The drugs are ranitidine, cimetidine, acetaminophen, aspirin, furosemide, bumetanide, piretanide, veralipride, sobrerol, penicillamine and doxycycline etc. Enterohepatic circulation-, two absorption site-, biphasic gastric emptying-, tissue deposition- and multi-fraction absorption theories have been suggested for the mechanisms of this phenomenon. Here, the theories were reviewed and critisized for their validity as a possible mechanism of the multiple peak phenomenon.