Earlier we have shown that administration to newborn rats of the pentose phosphate cycle inhibitor hydroquinone leads to a change in intensity and pattern of spontaneous periodic motor activity (SPMA) characteristic of early stages of development. The most typical was the disappearance of the rest period from the near-minute cycle "activity--rest" and the appearance of uninterrupted motor activity. In several cases, especially after 10 days of development, there was noted an enhancement in the SMPA pattern of the motor activity complexes following in the decasecond rhythm. In this study, on the 3-10-day old rats maintained under conditions of free behavior there was studied the blood glucose content in the animals at various periods of the activity-rest cycle. Apart from the SPMA phase, its composition (pattern) characterizing the maturity level and functional state of spinal motor centers was taken into account. In the 3, 7 and 10-day old rats at the rest period, the glucose concentration was established to differ depending on the motor activity pattern. In the case of the decasecond periodicity, it amounts to 5.7 +/- 0.2, 6.3 +/- +/- 0.3, and 7.7 +/- 0.3 mmol/l, while at the minute one--6.1 +/- 0.4, 7.8 +/- 0.3, and 7.8 +/- 0.1 mmol/l. At the moment of bursts of motor excitation, the glucose concentration falls to 5.2 +/- 0.1, 6.1 +/- 0.4, and 7.1 +/- +/- 0.3 mm at the decasecond and to 5.4 +/- 0.5, 6.7 +/- 0.2, and 7.6 +/- 0.3 mmol/l at the near-minute rhythm (for the 3, 7 and 10-day old animals, respectively). The results obtained on the 5-day rat pups differ qualitatively from those observed in other age groups. Thus, the glucose concentration at the rest period amounts to 6.8 +/- 0.2 at the decasecond and to 6.7 +/- 0.4 mmol/l at the nearminute periodicity. At the period of motor excitation accompanied by the presence of the decasecond activity rhythm, the glucose concentration falls to the level of 6.0 +/- 0.2 mmol/l by differing statistically significantly from the observed one in the rest state. In the case of the appearance of the minute rhythm, the glucose concentration amounts to 5.8 +/- +/- 0.3 mmol/l. The obtained data indicate that reproduction of the minute and decasecond rhythms recorded in composition of SPMA is accompanied by a change in the blood serum glucose level. The degree of a decrease of its concentration correlates with a certain activity rhythm: at the decasecond one the fall is 9, 13, 3, and 7 %, whereas at the minute rhythm--11, 13, 14, and 2% (for the 3-, 5-, 7- and 10-day old rats, respectively). In id to be noted that a certain effect on the activity pattern is produced by the degree of satiety of the rat pups, the gastric emptying being accompanied by an increase in expression of the decasecond rhythm and of brief jerks. Besides, there occur the significant seasonal oscillations of the blood serum glucose concentration in the newborn rat pups--at the summer period it is statistically significantly higher than at the winter-spring period.