Ten healthy volunteers received two sustained-release preparations as a single and multiple dose regimen in an open crossover study. Plasma theophylline concentrations were measured by an enzyme immunoassay. The limited fluctuation of the theophylline levels at steady state, with twice daily administration, clearly demonstrated the marked sustained release properties of both preparations. Results indicate similar properties for the two preparations. Significant correlations between the single dose period and steady state were found for Cmax and AUC (r = 0.76 and 0.87, respectively) with one formulation, whereas this was not the case for the other (r = 0.27 and 0.49). The daily dose necessary to keep the plasma concentration within the therapeutic range of 55-110 mumole/liter varied from 7.9 to 22.9 mg/kg. Only mild side effects were recorded, but they were not correlated to the plasma theophylline concentration.