1. Plasma pethidine levels have been monitored after the administration of 1.5 mg/kg intramuscularly to a group of young (under 30) and old (over 70) subjects. 2. Plasma levels were consistently higher in the old group, this was most marked for the first three hours but for most of the study there was a more than two-fold difference. 3. Differences in uptake from muscle and in metabolism were small and appeared unimportant. 4. Less pethidine was excreted in the elderly and this contributed to the overall differences in serum levels but was not important in explaining the marked disparity noted over the first three hours. 5. Red cell binding of pethidine by the young was much greater than by the old and if the differences in drug binding also applies to other tissues this would explain the high serum levels in the old and the increased incidence of side effects.