Hyperthermia is a body condition in which the body creates or absorbs more heat than it can release, leading to an increase in body temperature. Hyperthermia can occur even in relatively not very hot ambient conditions when the temperature is only around 21ºC, but as the temperature increases, so does the risk of heatstroke. Acute hyperthermia occurs in a short time interval, while subacute occurs during exposure to high ambient temperature for a longer period of time or during longer-term increased heat production in the body. The subject of our interest was the influence of high ambient temperature in different developmental periods in the white laboratory rat and its effect on the content of RNA in the adrenal tissue. We performed the tests on white rats that were divided into five experimental groups; a control group, which during the period of pregnancy, lactation and post-lactation up to the fiftieth day were at room temperature, a second group exposed daily for two hours to 40°C during pregnancy, and after hatching were at room temperature, a third group which only during lactation was two hours a day at 40°C, a fourth group two hours a day at 40°C in the post-lactation period and a fifth group exposed two hours a day at 40°C from birth to the fiftieth day. The working procedure was according to the protocol, and all animals were sacrificed at the end of the experiment. We performed the determination of RNA content in tissue according to Munro's method. The results show that high external temperature has a negative impact on experimental animals in all developmental stages, both in the ontogenetic and postnatal periods. Studies have confirmed the teratogenic potential of hyperthermia on them, regardless of what developmental period they are in. The constant increase in the mass of the adrenal glands is also the result of the continuous growth of the organism, but the dynamics of the growth and development of the adrenal glands in the different experimental groups shows a difference, depending on the developmental period in which they were exposed to high external temperature. The exception is animals exposed during pregnancy, where there is almost no effect of exposure. Data indicate that the adrenal mass during fetal development is much more developed, especially the cortex, compared to the rest of the glands and organs of the fetus. We can conclude that the most pronounced stimulating effect in the development of the adrenal gland is in animals that were continuously exposed to 40°C, from pregnancy to the fiftieth day. The increase in the mass of the adrenal glands is highly significant in the group of animals that received this treatment from birth to the fiftieth day of life. In order to see the change in the mass of the adrenal glands in relation to the growth of the organism, we also calculated the relative mass of the adrenal glands and the relative content of RNA in them. The obtained results are in accordance with the data of other authors. The values for the mass of the adrenal glands confirm the fact that under conditions of stress, the mass of this gland increases many times. This condition is probably due to its intensified function under stress, since the cells of the cortex of the glandulae suprarenales are target cells for ACTH in response to stress
