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SUMMARY Antipyrine elimination was studied in 29 patients with obstructive jaundice. Antipyrine half-lives calculated using plasma concentrations at four and 24 hours ('short antipyrine test') were significantly correlated with those calculated using six time points (p<0.001

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The effects of rifapentine (MDL 473) administration on hepatic mixed function oxidase activity in man have been investigated in six healthy volunteers. Administration of rifapentine (600 mg 48 h‐1) for 10 days resulted in a significant reduction in antipyrine half‐life (from 13.2 +/‐ 1.0 h to 7.7 +/‐ 0.4 h) and a corresponding increase in its total body clearance (from 41.8 +/‐ 5.5 ml min‐1 to 67.4 +/‐ 5.6 ml min‐1). Twelve days after stopping rifapentine administration, these values had largely returned to base‐line. 24‐Hour excretion of 6 beta‐ hydroxycortisol was significantly increased, by approximately three‐ fold, following administration of rifapentine for 10 days. Again, 12 days after stopping drug administration, 6 beta‐hydroxycortisol excretion had returned to pretreatment values. Clearance of antipyrine to its three oxidative metabolites was increased by rifapentine administration, although the increase for 3‐hydroxymethylantipyrine was not significant. The greatest increase (+140%) was observed for norantipyrine. Twelve days after the last dose of rifapentine, all values had returned to control levels. It is concluded that, like rifampicin, rifapentine is a potent inducer of mixed function oxidase activity in man and that the possibility of clinically significant drug interactions should be anticipated in the therapeutic use of this compound.