The pharmacokinetics of ethynyl estradiolsulfonate labelled with <sup>3</sup>H in the steroid nucleus and with <sup>14</sup>C in the isopropylsulfonic acid side chain was investigated in male minipigs. The plasma level after intravenous administration was interpreted according to a three-compartment model. After oral administration the first two phases were overlapped by the absoφtion period. The renal and fecal excretion of the two labellings were investigated. In urine the majority of the <sup>3</sup>H-labelled steroid was excreted in the unconjugated and in the glucuronide fractions. More than 70% of <sup>14</sup>C in urine was unextractable.