Acetaminophen is eliminated primarily by glucuronidation, thereby avoiding cytochrome P450-catalyzed bioactivation to a toxic reactive intermediate. Previous studies have shown that UDP-glucuronosyltransferase-deficient Gunn rats are more susceptible to acetaminophen toxicity than normal Wistar controls, from which the Gunn strain was derived. However, the Gunn and Wistar strains are not congenic, and differences in toxicologic susceptibility could be due in part to genetic differences other than UDP-glucuronosyltransferase activity. Accordingly, acetaminophen (750 mg/kg, ip) was administered to congenic RHA rats with normal (homozygous, RHA/++), moderately deficient (heterozygous, RHA/j+), and severely deficient (homozygous jaundiced, RHA/jj) activities of bilirubin UDP-glucuronosyltransferase. Acetaminophen metabolites were measured by high-performance liquid chromatography and production of the acetaminophen glucuronide conjugate was quantified by the area under plasma concentration-time curve (AUC) from 0 to 2 hr, standardized by the AUC value for acetaminophen in the same animal (glucuronidation ratio = AUC acetaminophen glucuronide/AUC acetaminophen). The 0- to 2-hr time period for AUC calculations was necessitated by the accumulation at later time points of glucuronide and sulfate conjugates in the plasma of animals experiencing severe nephrotoxicity. Acetaminophen bioactivation was quantified by the 24-hr urinary recovery of glutathione-derived conjugates. Hepatotoxicity and nephrotoxicity were assessed respectively by the peak concentrations of plasma alanine aminotransferase (ALT) and blood urea nitrogen (BUN). Glucuronidation of acetaminophen in RHA/jj rats (0.065 +/- 0.005) (mean +/- SE) was reduced 63% compared to the RHA/++ controls (0.17 +/- 0.01) (p < 0.05). RHA/jj rats demonstrated respective 230- and 7-fold increases in the peak plasma concentrations of ALT (17144 +/- 1014 vs 75 +/- 10) and BUN (128 +/- 23 vs 18.4 +/- 0.2) compared to congenic normal controls (RHA/++) (p < 0.05). Heterozygous animals (RHA/j+) demonstrated intermediary toxicity for both parameters (ALT = 2029 +/- 1581, BUN = 41 +/- 16, p < 0.05). Decreased glucuronide production correlated with elevations in ALT (r = -0.86, p < 0.001), while increased acetaminophen bioactivation correlated directly with both elevated ALT (r = 0.93, p < 0.001) and BUN (r = 0.83, p = 0.001). These results using congenic controls demonstrate that the enhanced susceptibility of UDP-glucuronosyltransferase-deficient rats to acetaminophen toxicity is due to decreased glucuronidation resulting in enhanced bioactivation, rather than to other unappreciated genetic differences.