The effects of concurrent administration of herbal tea prepared from dried flowers of Cassia auriculata or aerial parts of Cardospermum halicacabum and steady state serum levels of theophylline was investigated in Wistar rats. Results obtained demonstrate that a significant increase in the steady state levels of theophylline occur when this drug is administered concurrently with herbal tea prepared from either of the above plants. C. auriculata and C. halicacabum enhanced the steady state levels of theophylline by 32.5% (p < 0.02) and 48.2% (p < 0.02), respectively, when compared with the levels in animals receiving theophylline alone for the same time period. Herbal teas prepared from C. auriculata or C. halicacabum should therefore be avoided by patients treated with theophylline as these herbal teas have the potential to influence the bioavailability of the prescription drug.