Ninety-six subjects were asked to estimate durations of either "empty" or "filled" intervals during which they performed verbal tasks at three levels of difficulty. The verbal tasks were performed under three conditions of external rhythmic stimulation: fast, slow, and no external tempo. It was found that subjective time estimations were a decreasing function of task difficulty, and that durations for "empty" intervals were estimated to be longer than those for "filled" intervals. A relationship between external tempo and subjective time estimation was found. Longest time estimates were obtained under fast external tempo, and shortest time estimates were obtained under slow external tempo. Time estimates under the condition of no external tempo were found to be intermediate. The findings were interpreted as supporting a cognitive timer model of subjective time estimation.