Rana tigerina, native rice field frogs of Thailand, were used as experimenta animals. The ovaries obtained from 1- to 14-month-old female frogs were fixed in Bouins solution for light microscopy and with 2.5% glutaraldehyde in cacodylate buffer pH 7.4 for transmission electron micoscopy. The preovulatory oocytes were counted at monthly intervals under stereomicroscope. For studying the seasonal changes, ovaries from fully mature adult frogs (>12 months old) were obtained and counted every month by stereomicroscope. The developing oocytes can be classified into six stages based on the size, external features and internal morphology. The smallest stage I oocyte acquires translucent cytoplasm with 50-350 um in diameter. It also possesses smooth nuclear membrane and intensely basophilic cytoplasm after H&E staining, indicating the accumulation of free ribosomes. An opaque ring appears at the perinuclear zone of stage 11 oocyte which has the diameter ranges from 350-550 um. A few rows of Alcian blue positive vacuoles begin to develop at the periphery of the oocyte. Likewise the bundles of fine filaments also appear as a bright blue delicate band beneath follicular arch in this stage. The large central nucleoli are stained red. The dispersion of Alcian blue-positive vacuoles throughout cytoplasm occurs in the stage lll occyte (550-900 um). The important characteristics of this stage are the beginning of pigmentation and yolk deposition. Initial accumulation of yolk platelets begins at the periphery toward the central region. Of particular interest, stage IV oocyte (diameter 900-1190 um) appears light brown to brown color. Yolk platelets rapidly increase in number, replacing the central vacuoles and reflecting the high vitellogenic activity. Most of nucleoli become red when stained with Mallory trichrome dye. The polarity is externally visible in stage V (diameter 1100-1300 um) and Vl (diameter 1300-1500 um) oocytes. By this time, the animal hemisphere, where nucleus is situated, appears dark brown while the vegetal hemisphere is white which correlates with the amount of yolk platelet deposition and the depletion of pigmented granules. Cortical granules are apparent underneath the plasma membrane of stage V and Vl oocytes. During the development, it is Found that the ovaries of 1- to 4-month-old frogs contain only stage I oocytes. Ovaries of 5- to 6-month-old frogs consist mainly of stage I and 11 oocytes, while 7- to 10-month-old ovaries contain mainly stages l-lll oocytes. Stage IV oocytes begin to appear in 11-month-old frogs. Ovaries of 12-month-old frogs contain all stages of oocytes, however, only 66% of oocytes belong to stage Vi. In comparison, ovaries of 13- and 14-month-old contain 30.6 and 37.9% of stage Vl oocytes, respectively. During seasonal changes,it is observed that there is no stage Vl oocytes in the ovaries collected during November and February. Most of oocytes are in stage I (>40ฐ/0) and some are in stage ll and lll. In addition, a large number of degenerated oocytes (~25%) are observed. Stage Vl oocytes are present from March to October. The precentage of stage Vl oocytes are about 35.5, 37.2, 38.1, 37.1, 36.1 and 31.7% in March, April, May, June, July and August, respectively. In September the percentage of stage Vl oocytes is about 20% and decreases to 11.2% in October. It is concluded that the ovaries of R. tigerina become fully mature only after a period of one year. In fully grown frogs, the most fertile period when there are abundant stage Vl oocytes is during March to October.