Abstract <p>The sympathetic and parasympathetic regulation of heart activity and the thyroid gland is known. At the same time, information about seasonal changes in these processes in the conditions of the European North is practically absent. We have analyzed the seasonal changes in the levels of catecholamines, acetylcholine, thyroid hormones, and functional parameters of the cardiovascular system, taking into account the temperature of the atmosphere, as the leading factor determining the comfort of the external environment for human life activity. The same group of men (group I) (<i>n</i> = 20) was examined; practically healthy male population of Arkhangelsk (64°32′24″ NL) in seasons of the year with contrasting temperatures: summer (June) and winter (December) with the determination of the blood level of catecholamines in the blood (dopamine, adrenaline, norepinephrine), acetylcholine, and thyroid hormones (triiodothyronine (T3), thyroxine (T4), free triiodothyronine (fT3), free thyroxine (fT4), thyroid-stimulating hormone (TSH), and thyroxine-binding globulin (TBG)). The same group of men (group II) (<i>n</i> = 16) was examined in Syktyvkar (61°40′35″ NL). They were highly qualified cross-country skiers with a cyclic load on the body during the preparation for national competitions in winter, in December (cross-country skiing) and in summer, in July (roller skiing) with the determination of heart rate and blood pressure. The study of the autonomic nervous system was conducted on military personnel of the 2nd month of their compulsory service in Syktyvkar (group III) (<i>n</i> = 14), conscripted from central Russia, as well as on students (group IV) (<i>n</i> = 14), residents of Syktyvkar using the Ashner–Danyini test. It is shown that in December, compared with June, practically healthy men of group I have a decreased level of thyroid hormones, dopamine, and acetylcholine in the peripheral blood with an increase in low values of adrenaline and a tendency to increase the acetylcholine/adrenaline (ACh/adrenaline) and acetylcholine/dopamine (ACh/dopamine) indices, which is probably due to a decrease in the activity of the sympathoadrenal system. In men of group II involved in cross-country skiing, in winter, compared with summer, lower values of systolic-diastolic blood pressure are established and a pronounced tendency to slow down the heart rate is found. In practically healthy men of group III and group IV, mobilization of the body in response to physical activity is accompanied by an increase in the inhibitory effect of the vagus on the chronotropic function of the heart. It is assumed that this type of modification of the activity of regulatory systems is aimed at minimizing the energy costs of an organism living in seasonally unfavorable conditions of high latitudes, where the deficit of solar insolation in winter is accompanied by a cold atmosphere.</p>