There are a lot of factors affecting the release of hormones from the anterior part of pituitary gland and their interactions with other parts of the endocrine, nervous and immune systems. The special significance of the proestrus phase of the estrous cycle of the rat, during which LH and FSH levels increase, followed by ovulation is known. The short length of the estrous cycle and the well recognized sequence of vaginal lavage cytology make it useful for investigating the influence of a stressful environment on the reproductive function. Short duration and mild changes in environmental conditions is considered as a factor analogous to psychological stress. The study was undertaken to determine the effects of a short duration change in the ambient temperature and oxygenation (30 minutes) on the proestrus phase of reproductive cycle and on the repeatability and regularity of phases of the reproductive cycle of Wistar strain rats. The animals were kept under standard conditions and had food and water available ad libitum. The climatic chamber with automatically adjustable and monitored internal parameters (temperature, oxygenation, humidity) was used to develop stress conditions. An estimation of the vaginal lavage using the microscope was done to determine the estrous cycle. The animals were divided into 6 groups. On the day of experiment: the control group (CG) stayed in the climatic chamber for 30 minutes (ambient temperature 21 degrees C, normoxia - 21% O(2)), the five test groups (TG - I - V) remained in the climatic chamber for 30 minutes, in the established environmental conditions (I - 21 degrees C, 10% O(2); II - 10 degrees C, 21% O(2); III - 10 degrees C, 10% O(2); IV - 35 degrees C, 21% O(2); V - 35 degrees C, 10% O(2)). During the following days after the experiment, a microscopic estimation of vaginal lavage was collected over again. There were no changes of duration and sequence of the present estrous cycle and repeatability of the next cycles. Our results indicate that short duration change in the ambient conditions do not cause a disturbance in the hypothalamo - pituitary - gonadal axis, although it can activate adaptation mechanisms of the organism.