In a double-blind cross-over study, 20 children suffering from acute bronchoconstriction were treated with an interval of 90 min with placebo and 9.0 mg theophylline per kg given rectally in aqueous solution. Each child took part on 2 occasions. Fifteen min after both administrations of theophylline there was a significant increase in peak flow (PEF) as compared with placebo treatment (p less than 0.05). The increase in PEF was related to the serum level of theophylline, a significant bronchodilating effect was observed at about 7 mg/l. Substantial inter- and intrapatient variations were found in the improvement of PEF. The absorption of theophylline was rapid. The measured peak serum levels varied from 7.3 to 16.0 mg/l and occurred within 60 min in most patients. The intra-subject variations in the dose-to-dose absorption profiles were generally small and within a range acceptable for clinical use. The solutions allowed an accurate dosage calculation, did not initiate the urge to defecate, and were retained well.