The promoting effect of N-acyl derivatives of a collagen peptide which was fractionated from the hydrolysates of collagen on the rectal absorption of sodium ampicillin was studied. An increase in the blood level of ampicillin was observed in rabbits and rats following the administration of a sodium ampicillin suppository containing N-octanoyl(C8-CP), decanoyl(C10-CP), dodecanoyl(C12-CP), and stearoyl(C18-CP) derivatives of collagen peptide. The most efficient action was observed in the suppository containing C18-CP. Some factors influencing the promoting effect of the adjuvants on the rectal absorption of sodium ampicillin from solution were also studied in rats using in situ perfusion method and loop method. The promoting effect of the adjuvants on the rectal absorption of sodium ampicillin was markedly increased in the hypertonic solution. The promoting effect was also suppressed by the addition of calcium chloride to the solution administered to the rectal loop or to the perfusate in the in situ perfusion method. These results suggest that the tight junction of the epithelia of the rectum will be markedly influenced by the presence of the adjuvants, especially in the hypertonic solution. The adjuvant interaction with the calcium ions located in the rectal membrane may be involved in the enhanced absorption of sodium ampicillin.