This study examined daily and seasonal activity and thermoregulatory behaviour of the sleepy lizard, Tiliqua rugosa, a large, diurnally active temperate-dwelling Australian lizard, in the field and laboratory. Activity temperatures in the field were compared with those selected by lizards in laboratory thermal gradients in order to assess the extent to which endogenous versus exogenous factors contribute to seasonal variations in thermoregulatory behaviour. In the field, lizards are most active in late winter-spring (August-November), during which their activity varies from mostly unimodal on days of mild temperature to bimodal on hot days. In late spring-summer (November-January), activity is largely restricted to early morning, and at all other seasons sleepy lizards are rarely active. The winter-spring activity of sleepy lizards is constrained by low environmental temperatures, as lizards at these seasons have low body temperatures in the field but higher temperatures in laboratory thermal gradients. The lower temperatures selected in the laboratory in the summer-autumn months suggest the avoidance of high ambient temperatures and general inactivity in the field at these times. Thermal selection in the laboratory at the eight times of year tested showed that the phase of the minimum and maximum temperature selected and the amplitude of the rhythm of temperature selected varied continuously with the time of year. These daily and seasonal shifts in thermoregulatory behaviour may be regulated by endogenous physiological mechanisms coupled with seasonal ecological constraints such as food availability.