The widespread preference for rectal rather than oral administration of aminophylline was questioned and comparisons were made in 13 adults. Throughout four hours of testing, mean theophylline concentrations in the blood were higher with oral administration after fasting than with rectal administration by suppository or retention enema. In every patient, oral administration gave the highest blood theophylline concentration. After 500 mg of aminophylline in uncoated tablets was taken orally, a so-called therapeutic blood concentration developed in every patient.