Since the discovery of the structural formula of progesterone by Buttenandt and Westphal in 1934, many progestational steroid esters were synthesized and widely used for clinical purposes.<BR>Their biological activities were widely studied, but the works about their metabolism are only a few.<BR>This study was carried out on the hydrolyses of steroid esters including desoxycorticosterone acetate (DOCA), testosterone propionate (T-P), testosterone acetate (T-Ac), 1 - methyl-androsterone acetate (MA-Ac), pregnenolone enanthate (P-E), 17α-ethynyl-4- estrene-3β, 17β-diol-3-acetate (EED-3Ac), 6α-methyl-17α-acetoxy-progesterone (MAP), 17β-ethynyl-4-estrene-3β, 17β-diol diacetate (EED-DAc), 6-dehydro-6-methy1-17α-acetoxyprogesterone (Δ⁶-MAP) and 6-dehydro-6-chloro-17α-acetoxy-progesterone (Δ⁶-CAP).<BR>Identification and quantitative determination of these drugs and their metabolites were made with thin layer chromatography (TLC), INHA reaction and gas liquid chromatography (GLC).<BR>At first, in vivo investigation was made to examine the hydrolysis of EED-DAc and ENT-Ac. These steroid esters were administered orally to the female rabbits for 3 days in a total dose of 600 mg. After the administration, urine samples were pooled every 24 hours for 4 successive days. The urine was then incubated with β-glucuronidase and extracted twice with alkali and water followed by the evaporation.<BR>Isolation and identification of the urinary metabolites were carried out as described above and were identified as follows; 17α-ethynyl-5β-19-norandrostane-17β-ol-3-one (5β-ENT), 17α-ethylny-5β-19-norandrostane-3β, 17α-diol (5β-ENT-3β, 17α) and 17β-ethynyl-5α-19-norandrostane-3α, 17β-diol (5β-ENT-3β, 17β).<BR>To study the hydrolysis of steroid esters in the gastrointestinal tracts, steroid esters were administered orally through a polyethylene tube or directly to the small intestine at a proximal part of the duodenum of female rats.<BR>The stomach and small intestine were resected one hour after the administration. The metabolites were extracted with 20% acetone-water for 24 hours at room temperature and then finally extracted with pure acetone. The acetone extract was filtrated and evaporated. The aqueous residue was extracted with methylene chloride and washed with 1 / 10 N NaOH and water.<BR>The methylene cholride extracts were evaporated and examined on TLC and GLC.<BR>DOCA and esters of secondary alcohol at C-3 position were hydrolyzed in the stomach.On the other hand, in the small intestine, many kinds of steroid esters were hydrolyzed through to different extents. DOCA was hydrolyzed faster than the other compounds including steroid esters of secondary and tertiary alcohols.<BR>In the experiment of NaOH saponification, the hydrolysis of steroid esters were like the other organic chemical substances, controlled by their chemical structures.<BR>The rate of hydrolysis was found to be larger in the following orders : steroid esters of primary, secondary and tertiary alcohol.<BR>The experiment with female rat liver homogenate was carried out as follows : made abdominal incision, resectecl the liver after the perfusion with 1/15 phosphate buffer and homogenized the liver with buffer.<BR>The kidney homogenate was also prepared in the same way after exanguination.<BR>Both experimental results were similar to those found in the experiment of NaOH saponification.<BR>The fractionation of rat liver cell was done according to Hogeboom's method (1950) and the mitochondrial fraction was taken to prepare the acetone powder.<BR>Using the powder, a study was undertaken to investigate the hydrolysis of ENT-Ac in vitro, especially on the liniality between the volume of enzyme and the amount of ENT formed.