In a multiple dose cross-over experiment in 12 healthy male adults the bioavailability and sustained release characteristics of new once daily BY912 400 mg theophylline capsules (= B, Byk Gulden Research Laboratories, FRG) were studied using Theo-24 capsules (= T, Searle & Co., USA) as reference. Both products were given once daily for a period of 7 days as an 800 mg theophylline dose at 8 am, half an hour after a standardized breakfast. Theophylline concentrations in plasma were measured on days 1, 6, and 7 using high-performance liquid chromatography. Significantly better sustained release characteristics, resulting in longer plateau time (t75%, 11.6 vs 9.2 h on day 6 and 13.1 vs 8.8 h on day 7) and smaller per cent peak-trough fluctuation in the steady state (per cent PTF, 80 vs 103 per cent on day 6 and 66 vs 100 per cent on day 7), were found for B in comparison with T. The extent of absorption on both days, however, was smaller for B compared with T (relative bioavailability 84 per cent and 81 per cent, respectively). In conclusion, the absorption of theophylline from B resulted in a more extended shape of the plateau phase, indicating better sustained release characteristics. The extent of absorption, however, over the dosing intervals was more complete with T.