After oral administration of [14C]-labelled 3,4-dihydro-8-(2-hydroxy-3-isopropylamino)propoxy-3-nitroxy-2H-1-b enzopyran (nipradilol, NP, K-351) to spontaneously hypertensive rats (SHR) at doses of 3 and 30 mg/kg, the blood levels of radioactivity reached a peak at 1 h (197 and 1972 ng eq. NP/ml, respectively), whereas unchanged NP levels showed a peak at 30 min and Cmax values at both doses indicated a great difference (70-fold) in comparison with the dose ratio. Most of the radioactivity was excreted in urine and feces up to 72 h after administration. Large amounts of unchanged NP were distributed in the heart, aorta and vein which were considered to be target organs of NP. The half-lives of NP in those tissues, as well as in the blood, were short. The level of NP in the liver was low in comparison with the blood level. No significantly different levels of NP and its metabolites between SHR and normotensive rats were seen, except that the free type of 5-OH-DN was higher in the liver and urine in SHR.