Experimentally elevated testosterone has been shown to increase the daily activity period of free-ranging lizards, but it is unknown whether the effect depends upon additional factors present in nature (i.e., conspecific interactions) or whether it can be elicited in the laboratory. In a relatively simple laboratory environment, male Mountain Spiny Lizards (Sceloporus jarrovi) implanted with testosterone had significantly greater activity levels throughout the day than sham-implanted males. Differences were greatest during the morning and late afternoon, with testosterone-treated males more likely to be out earlier and to stay out later. This general result was repeated in both spring and fall experiments using separate groups of males. Although testosterone-treated males were more active (i.e., out in the open) than control males, there was not a significant difference in the frequency of movements (>20 cm), nor did the groups differ in selected body temperature in their home cages. Nonetheless, testosterone-implanted males lost significantly more body mass than controls over the course of the experiments. Because the effect of testosterone on daily activity period occurs in the laboratory as well as in the field, it is suggested that the effect is not dependent upon interactions with complex environmental stimuli.