The effect of aging on functional, biochemical, anatomical and molecular properties of endothelin (ET) receptors in bladder smooth muscle of the 3-week-, 3-month- and 22-month-old rats was examined using isolated muscle bath techniques, radioligand binding on membrane particulates and slide mounted tissue sections, and real-time reverse transcription polymerase chain reaction (RT-PCR). ET-1 induced significantly larger contractile responses in bladder dome muscle strips from 3-week- than from 3-month- and 22-month-old rats. The expression level of total ET receptors, determined by saturation binding experiments with [(125)I]ET-1, was higher in detrusor from 3-week- than 22-month-old rats. Inhibition studies with BQ123, a selective ET(A) receptor antagonist, indicated the predominance of the ET(A) receptor subtype and a similar proportion of ET(A) to ET(B) receptor subtypes in the rat detrusor at all ages studied. Autoradiographic data support the age-dependent decrease in the density of ET receptors and also indicate that the ET(A) receptor subtype is primarily located in the smooth muscle layer, whereas the ET(B) receptor subtype is located in both the urothelial and smooth muscle layers. Determined by real-time RT-PCR, ET 1, ET-3, ECE-1 and ET receptor subtype (ET(A) and ET(B)) mRNAs were shown to be higher in bladders of 3-week- compared to 3-month- or 22-month-old rats. This study indicates age-dependent alterations in the ET receptor system at both gene transcript and protein levels in the Fischer rat detrusor.