The oral bioavailability of triamterene in rats was investigated after its administration as a suspension in lipid and aqueous vehicles in addition to lactic acid solution. Triamterene is poorly soluble in both aqueous and lipid vehicles. A 1-ml oral dose volume showed that lipid vehicles may provide some enhancement of oral availability compared with aqueous suspensions. However, when the vehicle volume was reduced to a realistic dosage form volume for the rat, peanut oil and aqueous suspensions were indistinguishable from each other with respect to peak height, peak time, and overall bioavailability. For a given vehicle small vehicle volumes resulted in a better relative oral availability than did large vehicle volumes.