The acute effects of absorption enhancers on the nasal epithelium were investigated in vivo in rats. The following absorption enhancers were studied: dimethyl-β-cyclodextrin (DMβCD, 2% and 5% w/v), randomly methylated β-cyclodextrin (RAMEB, 2% w/v), hydroxypropyl-β-cyclodextrin (HPβCD, 5% w/v), sodium glycocholate (SGC, 1% w/v), sodium taurodihydrofusidate (STDHF, 1% w/v), L-α-lysophospatidylcholine (LPC, 1% w/v) and laureth-9 (1% w/v).The enhancer was administered to each nostril and after a period of 15 min the nasal cavity was washed out by a perfusion via an oesophageal canula. The release of the marker compounds protein, cholesterol, acid phosphatase from the epithelium after a single nasal dose of an absorption enhancer was determined using a nasal lavage technique. The effect of the absorption enhancer on the nasal epithelium was determined from the release of several marker compounds in the lavage. Additionally, the increase of the permeability of the nasal epithelium, as a result of the administration of an absorption enhancer, was measured by the leakage of a systemic tracer from the capillaries into the nasal cavity. The systemic tracer, fluorescence labelled albumin (FITC-albumin), was administered intravenously to the rat 5 min prior to the nasal dosage.The effects of the absorption enhancers could be classified into four categories using ANOVA and Least Significance Difference testing. The first group consisted of HPβCD (5%), DMβCD (2%) and RAMEB (2%) and was not different from the control (physiological saline). For the second category, DMβCD (5%), effects were significantly higher than for the control. The third category, SGC (1%), was more active than DMβCD (5%) but less active than the last group. The fourth, most damaging, category consisted of STDHF (1%), laureth-9 (1%) and LPC (1%). Administration of the enhancers of the last group resulted also in release of acid phosphatase, indicating that severe cell damage occurred. The release of cholesterol was dependent on the interaction of the absorption enhancers with cholesterol. The amount of cholesterol released by laureth-9 and LPC was the largest.The results of this in vivo study are in agreement (i.e. similarity in rank order) with morphological and ciliotoxicity studies of nasal absorption enhancers, demonstrating that this in vivo model can be a valuable tool to classify nasal absorption enhancers according to their effects on the rat nasal epithelium.