BACKGROUND: We hypothesized that the 5-oxo-1,2,4-oxadiazole moiety of azilsartan (AZL), which represents a small difference in the molecular structures of AZL and candesartan (CAN) [angiotensin II type 1 receptor (AT1R) blockers], may be responsible for the molecular effects of AZL. METHODS: We examined the binding affinities of AZL and CAN to AT1R, along with their ability to block receptor activity. A competition binding study, inositol phosphate (IP) production assay and extracellular signal-regulated kinase (ERK) assay were performed using wild-type (WT) and mutants AT1R-transfected cells. RESULTS: The binding affinities of CAN and AZL were reduced by > 5-fold for Y35F, W84F, R167K, K199Q and I288A compared with WT. In addition, AZL showed a > 5-fold reduction in its binding affinity to V108A. CAN and AZL exhibited > 20-fold and > 100-fold reductions in binding affinity to R167K, respectively. The loss of binding affinity of AZL to R167K was greater than that of CAN. CAN-7H exhibited a > 10-fold reduction in binding affinity to R167K compared with CAN. On the other hand, the binding affinity of AZL-7H to R167K was comparable to that of AZL. While 10-6M CAN and CAN-7H partly blocked Ang II-induced IP production in R167K, 10-6M AZL and AZL-7H did not. In addition, 10-6M CAN, but not 10-6M AZL, partly blocked Ang II-induced ERK activation in R167K. CONCLUSIONS: The interaction between 5-oxo-1,2,4-oxadiazole in AZL and Arg167 in the AT1R appears to be more important than the interaction between the tetrazole ring in CAN and Arg167.