To study the factors influencing nasal absorption of a model pentapeptide, plasma levels of total radioactivity were determined following the administration of [3H]tyr-leucine enkephalin to rats intravenously, intranasally alone, and intranasally in the presence of puromycin. The major pathway for transport of radioactivity into the blood from the nasal cavity appeared to be hydrolysis of [3H]tyr-leucine enkephalin to [3H]L-tyrosine, followed by absorption of [3H]L-tyrosine. When puromycin was added to the nasal solution in concentrations at which the in vitro hydrolysis of leucine enkephalin was completely inhibited, the appearance of radioactivity in the plasma was slowed, but plasma concentrations of radioactivity eventually reached levels comparable to those observed in the absence of puromycin. In view of the inhibitory effect of puromycin on the hydrolysis of leucine enkephalin, it was assumed that a significant fraction of the [3H]tyr-leucine enkephalin was absorbed intact in the presence of this substance. However, an assay method for intact leucine enkephalin in plasma is needed to confirm these preliminary observations.