This study tests the assumptions of need for stimulation theory. According the main hypothesis of this theory, the stimulus seeking activity of an organism in an unfamiliar environment is affected by two main temperamental traits: emotional reactivity and need for stimulation. In a familiar setting, the influence of emotional reactivity disappears, while the need for stimulation persists. Two experiments were run in which animals' emotionality was manipulated by means of presence or absence of handling and the level of environmental stimulation was manipulated by varying the intensity of light to which the animals were exposed. Sixty male Wistar rats were tested in the first experiment. Stimulus seeking activity was registered in the Skinner-type chambers where animals could switch the light on by every head dip into one of two holes, the so-called experimental hole. Animals were tested in five 30-minute sessions repeated every 48 hours. As predicted, the effect of emotionality on exploration emerged at the beginning of the experiment (handled rats demonstrated a stronger preference for the experimental hole), whereas the effect of the level of environmental stimulation on the total number of head-dips emerged in all the experimental sessions. The second experiment involved 40 rats and followed a similar design, but the stimulus seeking activity was measured in the situation where the animals could switch the light off by dipping their heads into the experimental hole. Contrary to predictions, the experimental factors had no significant effect on the animals' stimulus seeking activity. Only the results of the first experiment confirm the assumptions of need for stimulation theory.