Enhancer-based drug preparations allow absorption of peptide drugs. We investigated the reversibility with time of nasal absorption of human growth hormone (hGH) induced by the absorption enhancers didecanoylphosphatidylcholine (DDPC) and a-cyclodextrin (a-CD). Rabbits were dosed intranasally with enhancer in the absence of hGH at time -3, -1, -0.5 and 0 h. At time zero the same groups of rabbits were dosed with a hGH powder devoid of the enhancers. Values for plasma hGH AUC and C(max) were estimated in order to measure the degree of absorption, and rabbits receiving hGH together with enhancers were used as positive references. With an enhancer preparation of 8% DDPC and 30% a-CD, the hGH AUC and C(max) values showed a significant time-dependent decrease after enhancer administration. This may indicate recovery of the enhancer-induced mucosal leakiness as well as clearance of the enhancers from the tissue. Similar findings were obtained with a-CD alone, whereas a preparation containing only DDPC showed no recovery of leakiness. The effect of a-CD was also investigated in vitro in rabbit nasal mucosa mounted in Ussing chambers. Incubation for 15 min with 1, 3 and 8% a-CD induced a concentration-dependent decrease of the potential difference, the short circuit current and the resistance suggesting impaired sodium transport and loosening of tight junctions. After washout of a-CD, this effect was completely reversible for the low concentrations of a-CD, but not for the 8% a-CD. These results show that a-CD alone, or in combination with DDPC, has properties as a reversible enhancer for nasal absorption of hGH in vivo.