The influence of different suppository bases on the dissolution, and the bioavailability of aspirin suppositories in rabbits and humans was investigated using Witepsol H15 (WIT), WIT-Tween 80 (TWE), WIT-sodium lauryl sulfate (SLS), polyethylene glycol (PEG), hollow WIT (WIT-HOLL) and capsule incorporated into WIT (WIT-CAP). The results obtained were as follows: 1) Dissolution rates of aspirin suppositories with different bases in distilled water were faster in the order of WIT-TWE > WIT-SLS > PEG > WIT-HOLL > WIT > WIT-CAP. 2) The maximum blood levels (C_(max)) of aspirin in rabbits and humans were highest in WIT-TWE and WIT-SLS bases, but C_(max) from WIT base was lower than that in oral administration of aspirin suspension. 3) The times reaching the maximum blood levels (T_(max)) in rabbits were 1hr for oral administration, 1.5-2.5 hr for WIT-TWE, WIT-SLS, PEG, and WIT bases, and 2.5-4.0 hr for WIT-HOLL and WIT-CAP bases, but T_(max) in humans were 1hr for oral administration and WIT-TWE base, and 2-4 hr for WIT and WIT-HOLL bases. 4) Relative bioavailability (RBA) of aspirin suppositories in rabbits was higher in WIT-SLS, WIT-TWE and PEG bases than that in oral administration, and RBA of aspirin suppositories in humans was higher in the order of WIT-TWE > PEG > WIT-HOLL > oral > WIT bases tested. 5) Good correlation between dissolution rates and C_(max) was obtained : y = 0.60x + 32.23 (r = 0.96) for rabbits, and y = 0.60x + 35.74 (r = 0.97) for humans.