The bioavailability of orally administered hydralazine was assessed in 4 healthy subjects after separate administration of a single oral or intravenous dose (0.3 mg-kg-1). Comparison of the areas under the serum concentration-time curves showed that 26-55% of the oral dose was available to the systemic circulation as unchanged drug. The 0-24 h excretion of the drug in urine was rapid: 11.4-14.1% of the dose after intravenous administration, and 2.0-3.6% after an oral dose. Acetylation of hydralazine leads to formation of 3-methyl-s-triazolo-3,4, aphthalazine (MTP) and a gas-liquid-chromatographic method for its measurement in urine was developed. After oral and intravenous administration, 0.8-1.2% and 1.4-2.3% of the dose, respectively, were recovered within 24 hours from urine as MTP. After oral administration there was a relative increase in the amount of MTP in every subject, which indicates route-dependent metabolism. The lower bioavailability of oral hydralazine could be explained in terms of first-pass metabolism.