1 A sensitive, stereospecific assay using gas chromatography-mass spectrometry (GC/MS) was established to measure the concentrations of the enantiomers of ibuprofen in small volumes (50 gl) of blister fluid.2 The concentrations of the enantiomers in blister fluid, assessed in eight patients, were similar to those in synovial fluid, both fluids behaving as peripheral compartments with respect to plasma.3 The mean rate constants of transfer of R-ibuprofen into (0.14 ± 0.06 h-1) and out of (0.20 ± 0.04 h-1) blister fluid were not significantly different from those for synovial fluid (0.19 ± 0.12 h-1, 0.34 ± 0.11 h-1, respectively). Similarly, the mean rate constants of transfer of S-ibuprofen into (0.22 ± 0.07 h-1) and out of (0.27 + 0.08 h-1) blister fluid were not significantly different from those for synovial fluid (0.29 ± 0.10, 0.36 ± 0.11 h-1). However, the correlations were poor between the transfer constants for each of the enantiomers between plasma, and both blister and synovial fluid (P > 0.2). 4 The complex rate constant of transfer of S-ibuprofen into blister fluid (0.22 + 0.07 h-1) was greater than that of R-ibuprofen (0.14 ± 0.07 h-1), which may be explained by the lesser protein binding of the S-enantiomer. 5 The mean AUC of the S-enantiomer was greater than of the R-enantiomer in both blister (116 ± 43 mg 1-' h, 73 ± 32 mg 1-l h, respectively; P < 0.05) and in synovial fluid (110 ± 28 mg 1-' h, 56 ± 8 mg 1-F h, respectively; P < 0.01). The difference between the mean AUC values of the enantiomers in synovial fluid vs blister fluid was not significant. 6 Despite similar concentration-time profiles, and mean pharmacokinetic parameters, blister fluid was not a good model for predicting the kinetics of the enantiomers of ibuprofen in synovial fluid in individual patients. However, the blister as a peripheral compartment, better simulates the time course of concentrations in synovial fluid than does plasma, and consequently drug concentrations therein might better relate to clinical efficacy than do those in plasma.