The aim of this study was to analyse the pattern of activation and habituation of youngsters with head injuries depending on the injury's gravity. Nine patients were examined with decerebration after an apallic syndrome; the control group contained ten patients with slight cerebral contusion. The complex test stimulus (optical and acoustical stimulation requiring a reaction) was a phase of a daily, continuous 4-min load with the 'Wiener Determinationsgerät' (Schuhfried). The frequency of heartbeat was registered for 4 (8) days. We analysed the physiological and psychosubjective estimation as well as the psychophysical level of measuring to obtain concrete steps for rehabilitation, depending on the possible interactions. All variables demonstrated a definite distinction between the two groups of patients. Analysis of heartbeat frequency showed that, in spite of similar initial levels of activity, 'initial sensitization' appeared only in the control group; it diminished with habituation and repetition of the stimulus. The descriptive analysis of the HR development in the form of a mathematical exponential function showed that there were three relatively consistent types of reaction: sensitization, habituation, and indifference (nonresponder). The habituation reaction was found only in the control group, indifference only in the group of apallic patients. The analysis of reciprocal action of different levels of measurings shows a certain disorder in the functional systems of the apallic patients, as described by Luria (1976). After the test stimulus the sensomotor and physiological systems do not interact optimally in comparison with patients with slight head injuries. The resulting steps for rehabilitation are discussed.