Twelve medical students were shown two series of ten pictures of orange circles followed by pictures of nude females, alternating at 1-min intervals with ten pictures of blue triangles followed by pictures of nude males. All were shown for 10 sec. In one series, the pictures were moving and were inserted into a travelogue-type film; in the other, they were still, and still pictures of landscapes were shown in the intervals. Six students were randomly allocated to watch the moving sequences first; the other six watched the still sequences first. The summed penile volume increases of the students to the moving pictures of the women were significantly greater than those to the still pictures. Summed conditioned penile increases were also greater to the pictures of circles preceding these moving pictures compared with those to circles preceding the still pictures. Only the former conditioned responses correlated significantly with the unconditioned penile responses on which they were based. It was concluded that conditioned penile volume increases could be established more satisfactorily using moving rather than still pictures of female nudes as unconditioned stimuli. The summed penile response to ten such still stimuli nevertheless provided a reliable measure of the subjects' responsiveness.