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11634 (Youngdale, Duncan, Emmert & Lednicer, 1966), is orally active as an antifertility agent in the rat (Duncan, Johnston & Lyster, 1966) but not in the monkey (Meyer & Wolf, personal communication). This difference could reflect differences in drug availability following oral administration or could be due to the mechanism of action of U-11634. Species-specific responses have been reported for the antifertility diphenylindenes (Duncan, Stucki, Lyster & Lednicer, 1962; Duncan & Lyster, 1962) and diphenyldihydronaphthalenes (Duncan, Lyster, Clark & Lednicer, 1963). Accordingly, a comparison of drug availability based on serum drug levels has been made following oral adminis- tration of U-11634 to the rat and monkey. Eight groups of five female Sprague-Dawley rats, each weighing 219 to 231 g, were employed. One group served as a control. Each member of the other seven groups received 25 mg of U-11634 as a crystalline suspension in 1 ml of sterile aqueous 0\m=.\25%methylcellulose by stomach tube. At appropriate times (Table 1), blood was removed by heart puncture from ether-anaesthetized animals and serum collected. Two male cynomologous (Macaca irus) monkeys weighing 4 and 5 kg, respectively, received 200 mg/kg of U-11634 as the crystalline powder in a gelatin capsule orally. At appropriate times (Table 1), blood was removed from the femoral veins and serum collected. One-ml serum samples were mixed with 1 ml of 0-1 N-NaOH and extracted with 25 ml of chloroform. Twenty-ml aliquots of the chloroform extracts were evaporated to dryness at room temperature and the resulting residues dissolved in 5 ml of 95% ethanol for ultra-violet spectrophotometry. Serum drug concentrations were calculated from the observed absorbance at the 243- µ maximum for U-11634, after correction for the response found for pre-treat¬ ment serum samples (serum blanks). Under these conditions, recovery of U-11634 from serum was quantitative (98-3 + 3-3%) and mean serum blanks found for rats and monkeys were equivalent to 3-2 and 2-4/tg of U-11634/ml, respectively. Serum levels of

COMPARATIVE SERUM LEVELS OF AN ANTIFERTILITY OXAZOLIDINETHIONE (U-11634) IN RATS AND MONKEYS