Recent studies suggest that disposition of talinolol (TAL) is affected by the efflux pump, P-glycoprotein (P-gp). However, co-administration of verapamil (VER), a known P-gp inhibitor, and TAL in humans resulted in decreased TAL plasma levels instead of a predicted increase resulting from VER-mediated inhibition of intestinal P-gp. In the current study, we show that TAL is actively transported across polarized epithelial cells expressing high levels of P-gp (LMDR1). Moreover, when TAL was orally administered to mice, plasma levels were nearly 3-fold higher in the mdr1a/1b-deficient (-/-) mice, compared to their wildtype (+/+) counterpart. However, co-administration of TAL (16 mg.kg^-1) to (+/+) and (-/-) mice resulted in a significant reduction in TAL plasma levels (ng.ml^-1 at 1 hr, mean±SD) in both groups: TAL TAL+VER (+/+) 361±155 96±77 (-/-) 1054±349 575±283 These findings suggest that inhibition by VER of a TAL intestinal uptake transport system(s) may overshadow the effects of VER on P-gp.