In a crossover design, cats were fed a canned heat-processed diet (18 g dry matter/kg initial body wt) either with (+) or without (-) antibiotics [a mixture of penicillin G, procaine (25 mg/18 g diet) and tetracycline (50 mg/18 g diet)]. The (-/+) group received no antibiotics during the first 5-wk period and received antibiotics during the second 5-wk period; the (+/-) group received the reverse. Plasma, whole blood, urinary and fecal concentrations of taurine, fecal bile acid excretion and cholyltaurine hydrolase activities were measured. Consumption of antibiotics for 5 wk resulted in a lower rate of depletion of plasma taurine. Taurine concentrations decreased more over the first 5 wk in cats in the (-/+) group than in cats in the (+/-) group [from 116 +/- 26 to 26 +/- 6 mumol/L (-/+) and from 109 +/- 6 to 77 +/- 7 mumol/L (+/-) for plasma, and from 546 +/- 8 to 292 +/- 29 mumol/L (-/+) and from 560 +/- 11 to 431 +/- 20 mumol/L (+/-) for whole blood]. Urinary total taurine excretions during the 5th week were 54 mumol/d for the (-/+) group and 135 mumol/d for the (+/-) group (pooled SEM, +/- 13). Fecal total taurine excretions during the 5th week were 184 and 53 mumol/d for the (-/+) and (+/-) groups, respectively, (pooled SEM +/- 9). Most of the fecal taurine was unconjugated (free). Fecal bile acid excretions during the 5th week were 235 +/- 18 and 106 +/- 11 mumol/d for the (-/+) and (+/-) groups, respectively. Dietary antibiotics suppressed fecal cholyltaurine hydrolase activity of cats. Fecal cholyltaurine hydrolase activities during the 5th week were 279 +/- 54 and 42 +/- 10 nmol cholic acid released.min-1.g dry feces-1 in the (-/+) and (+/-) groups, respectively. After the crossover, mean values for the groups were reversed, showing that the observed changes were due to the antibiotic treatment. These results support the hypothesis that the dietary taurine requirement of cats is largely determined by the extent of microbial degradation of taurine in the gastrointestinal tract.