The timing of drug ingestion over 24 hours was assessed in 58 outpatients taking drugs three times daily. The mean time between 1st and 2nd dose was 5.7 hours, 2nd and 3rd 6.1 hours and between 3rd and 1st of the next day 12.2 hours. Based on a simulation of hydralazine pharmacokinetics, such irregular administration would increase peak levels by 10% and decrease trough levels by 64% compared with regular 8-hourly doses.