The short-term effects and long-term consequences of sodium nitrite-induced hypoxia on spontaneous behavior of adult male rats were studied. A short-term suppression of locomotor, exploratory and grooming activities in 3-month-old animals after single NaNO2 does (55 mg per kg, s.c.) was followed by the recovery of normal values within 24 h. A marked decrease in locomotor and exploratory activity as well as of investigatory time combined with an increase of time spent in relaxation patterns was found in 6-month-old males at day 7 after double NaNO2 administration. A more rapid habituation during repeated exposure of 3-month-old males into the same experimental box was found beginning on day 10 after three-fold NaNO2 administration. We suggest that the delayed behavioral deficits could be due to the secondary structural changes developing in the central nervous system over several days after the NaNO2 insult. Alaptide, a dipeptide derived from Pro-Leu-Gly-NH2, administered for five days in a preventive-curative way ameliorated the deficits in spontaneous behavior consequent to the NaNO2 effect. At the moment it is not possible to fully understand the mechanism of action which is responsible for the beneficial effect of alaptide.