The degree of stimulus control of free-operant avoidance was investigated through two probe procedures that assessed stimulus control within a single session. Twenty-five rats responded under a signaled free-operant avoidance contingency. Each response postponed the 10-s preshock (warning) stimulus for 15 s (if it occurred in its absence) or terminated that stimulus (if it occurred in its presence). For seven rats the discrimination procedure used visual stimuli (houselight-darkness); 18 rats were trained with auditory stimuli (tone - no tone). After 30 consecutive baseline sessions a probe procedure was imposed on the baseline conditions in every second or third session. The probes consisted of varying the response-preshock stimulus (RS1) interval during sample periods of the session. Both on baseline and on probe sessions stimulus control depended on the modality of the discriminative stimuli. The auditory group was more resistant to changes in the RS1 interval during the probe sessions. Results clearly show that the two probe procedures used were successful in detecting underlying temporal discrimination helping in exteroceptive stimulus control of avoidance responding.4214155