The investigation was performed on the medial (MMS) and lateral (LMS) magnocellular subdivisions of the hypothalamic paraventricular nuclei (HPN). The histochemical activity NO synthesizing enzyme nitric oxide synthase or NOS whose histochemical marker is NADPH-diaphorase (NADPH-D), immunocytochemical content of oxytocin (OXY), vasopressin (VP) and nucleoli sizes (squares) were studied in the mature male rats under experimental reconstruction of the both micro- and macrogravity, which are factors of the gravity field changes acting to the body during the space flight. Two experimental effects were used: B--tail suspending (imitation of the microgravity effects), C--centrifugation at 2 G (imitation of the macrogravity effects). The effect durations were designed as a time period when body is mostly affected by (1 day) and adapted (15 days) to the stress. There were 6 animal groups. 1--B(15 days), 2--B(15 days) succeeded by C(1 day), 3--B(15 days) succeeded by C(15 days), 4--C(1 day), 5--C(15 days), 6--intact animals. The histochemically and immuno-cytochemically stained neurons developing the high, moderate and small reaction intensity were counted in serial HPN sections under the light microscope and the results obtained were transformed to percent neuron contents. The nucleoli squares were examined by using the TV analyser. The histochemical staining intensity of NADPH-D in MMS is enhanced in the animals of the groups 1-4; the number of NADPH-D staining neurons with high enzyme activity was increased in 8-14 times. In the animals of group 5 the NADPH-D activity did not differ from the intact animals. The number of MMS neurons with high OXY immunoreactivities was increased up to 1.5-1.7 times in groups 1-5 if compared to those of intact controls. VP-positive neurons of LMS developed the similar increase in number of the high staining neurons in experimental animals as well as OXY-positive neurons of MMS. The nucleoli enlargement was observed in MMS (in 1.3-1.5 times) of groups 1-5 (insignificantly in group 5) and in the most magnocellular neurons LMS (in 1.5-1.7 times) of group 2-5 except group 1 where nucleoli were insignificantly decreased. The nucleoli sizes of group 4 were more than group 5. So the hypothalamo-neurohypophyseal system was activated in the animals subjected of the earthly correlates of micro- and macrogravity. The data obtained suggest involvement both the nonconventional neurotransmitter NO and stress-related peptides OXY and VP in the mechanisms subserving adaptation to the extreme factors by what a human has to be faced with during the space flight.