In a time production task, the participants' activation level and attention devoted to time were manipulated respectively by means of click trains delivered at 2 different intensities during the task and by introducing a concurrent reaction time task. Activation level is classically considered to affect the rate of an internal pacemaker, whereas the way attention affects time estimation is a matter of debate. Three models that differ as to the effect of attention were evaluated. Predictions on the interaction pattern between activation and attention were derived for each of the 3 models. When manipulated jointly, these 2 factors proved to be independent, as they had additive effects on the performance. This finding suggests that the activation level affects the pacemaker rate, whereas the attention level affects an accumulation process by directly acting on a switch functioning in an all-or-none fashion.