The effects of several prospective absorption enhancers were assessed on the nasal absorption of biosynthetic human growth hormone (hGH) in the rat. These enhancers function by alternative mechanisms that include enzyme inhibition, reduction in mucus viscosity, and enhancement of membrane fluidity. The levels of plasma hGH achieved were determined by an enzyme-linked immunosorbent assay. The increase in peak height was calculated relative to nasal administration of hGH alone without any enhancers and the relative bioavailability was calculated with reference to subcutaneous injection data. A lysophospholipid, lysophosphatidylcholine, gave the highest peak concentration, with an increase in peak height of 450% and a relative bioavailability of 25.8%. However, the greatest increase in AUC (291%) was achieved with the aminopeptidase inhibitor, amastatin, which gave a relative bioavailability of 28.9%. A mucolytic agent, N-acetyl-L-cysteine, and a transmembrane fatty acid transporter, palmitoyl-DL-carnitine, were also found to promote the nasal absorption of hGH in this model, with relative bioavailabilities of 12.2 and 22.1%, respectively. Bestatin, an enzyme inhibitor, was not an effective absorption enhancer for hGH in this model.