The nine years old giant panda YAN YAN was received in April 1995 on loan for 5 years to the Berlin Zoo. Urine samples were collected daily or every second day from April 1995 to June 1996 in order to follow up sex hormone secretions and ovarian activities. Conjugated steroids were hydrolysed, extracted and measured with two enzyme immuno assays (EIA) being specific for either total oestrogenes or pregnandiol. The evaluation of the hormone secretion pattern yielded the following results: There is a significant synchronous cross correlation between estrogen and progestin metabolites secretion indicating its simultaneous synthesis. In addition, we found a regular increase and decrease of both hormones with a 13 days interval. This secretory pattern indicates repeating development and atresia of follicle cohorts with a cycle length of about 13 days. Only a single period of slightly elevated oestrogen synthesis was monitored in Feb. 96 without any signs of oestrus. Obviously the stimulation of ovarian function was insufficient for complete ovulation and corpus luteum formation.