Our laboratory reported previously the in vivo absorption of salicylate and antipyrine from the urinary bladder of healthy rats. The present study extends these findings to examine the effect of interstitial cystitis on bladder permeability to drugs by comparing the bladder absorption of salicylate in healthy cats and cats with feline interstitial cystitis (FIC). Interstitial cystitis is a lower urinary tract disease and may involve inflammation of the bladder. A 25-mg/kg dose of sodium salicylate was instilled into the bladder and a 6-micrograms/kg i.v. dose of [14C]salicylic acid was administered concomitantly to determine systemic clearance in individual animals. In healthy and FIC cats, the clearance was 0.3 and 0.1 ml.min-1.kg-1, and the bioavailability of the intravesical dose was about 1 and 9%, respectively. The apparent bladder absorption rate constant and the peak plasma concentration in FIC cats were 7 to 8 times that in healthy cats. These data indicate a significantly higher bladder permeability to salicylate due to interstitial cystitis. In order to elucidate the cause of the lower clearance in FIC cats, a second study compared the disposition of a 2-mg/kg i.v. dose of salicylate in healthy and FIC cats. It was found that the lower salicylate clearance in FIC cats was due in part to a decreased metabolism in the disease state and also involved slower drug elimination at the higher plasma concentrations secondary to the more extensive absorption of the intravesical dose.(ABSTRACT TRUNCATED AT 250 WORDS)