31 healthy male (n = 17) and female (n = 14) volunteers, aged 20 to 45 years, were divided into 4 groups and received on 3 separate occasions either: paracetamol (acetaminophen) 650mg intravenously (n = 9); alprazolam 1mg orally (n = 7); antipyrine (phenazone) 1g intravenously (n = 8); or lorazepam 2mg intravenously (n = 7). Doses were administered prior to influenza vaccine (0.5ml, intramuscularly) and at 7 and 21 days post-vaccination. The overall differences among the 3 trials in clearance of antipyrine were of borderline significance (p less than 0.0611), with a trend towards reduced clearance in both of the post-vaccination trials. There were no overall differences observed in the elimination half-life of antipyrine, nor were there significant differences between trials in cumulative urinary excretion or fractional recovery of intact antipyrine, 4-hydroxyantipyrine, norantipyrine, or 3-hydroxymethyl antipyrine. For paracetamol and alprazolam, there were no significant differences among the 3 trials in any of the kinetic variables. The elimination half-life of lorazepam varied significantly among trials, but differences were small and not systematic. Lorazepam clearance did not vary significantly among trials. Thus, clearance of drugs which undergo hepatic conjugative reactions such as glucuronidation and sulphation are unlikely to be affected by the coadministration of influenza vaccine. Furthermore, not all drugs which are biotransformed by hepatic microsomal oxidation necessarily have impaired clearance due to coadministration of influenza vaccine.