Intestinal absorption of two oxovanadium complexes, vanadyl acetylacetonate (VO(acac)2) and bis-(maltolato)-oxovanadium (VO(ma)2), has been compared using Caco-2 monolayers as a model system. The two compounds are similar in chemical structures but different in glucose-lowering effects. Our experimental results show that they are both transported via passive diffusion with apparent permeabilty coefficients (apical→basolateral) of (82.0 ± 6.7) ×10-7 and (14.6 ± 0.7) ×10-7 cm·s-1, respectively. This suggests that absorptivity of VO(acac)2 is much higher than that of VO(ma)2. This difference may be related to the metabolism of either compound, or its ligand, or both in the course of the transport. However, This difference in absorption will cause the great difference in bioavailability, which might account for better efficacy of VO(acac)2 than VO(ma)2 as the insulin-mimic agent. Intestinal absorption of two oxovanadium complexes, vanadyl acetylacetonate (VO (acac) 2) and bis- (maltolato) -oxovanadium (VO (ma) 2), has been compared using Caco-2 monolayers as a model system in chemical structures but different in glucose-lowering effects. Our experimental results show that they are both via via passive diffusion with apparent permeabilty coefficients (apical → basolateral) of (82.0 ± 6.7) × 10-7 and (14.6 ± 0.7) × 10 -7 cm · s -1, respectively. This suggests that absorptivity of VO (acac) 2 is much higher than that of VO (ma) 2. This difference may be related to the metabolism of either compound, or its ligand, or both in the course of the transport. However, this difference in absorption will cause the great difference in bioavailability, which might account for better efficacy of VO (acac) 2 than VO (ma) 2 as the insulin-mimic agent.