Eight rats responded for food in a discrete-trial discrimination procedure (Go/No-go discrimination task). The subjects' head entries to the feeder were reinforced whenever the discriminative stimuli ('Go' signal) were illuminated. Two lights located at opposite sides of the experimental chamber served as discriminative stimuli, and the 'No-go' component was signaled by a time-out (TO) of variable durations. The subjects' global responding in the discrimination task were described at different levels of analysis, as well as the distribution of other behaviors during the 'No-go' periods. Each of eight subjects learned to differentially respond to stimuli, showing short latencies from the S+ onset; nevertheless, commission errors differed among subjects. The video-recording analysis revealed that the time-allocation of other behaviors diverged among subjects. Specifically, the subjects with numerous commission errors engaged less time in alternative activities (i.e. nosing the floor) than did the subjects with less commission errors. Findings suggest that in a simple discrimination task, the allocation of activities are relevant to a comprehensive understanding of stimulus control and relative phenomena such as behavioral contrast.