Rats were trained to bar-press on a differential reinforcement of low rate 16 sec (DRL 16) schedule for water reinforcement. On alternate days, rats were tested in the presence or absence of a light which signaled the availability of reinforcement. Rats were allowed to bar-press on either of two levers (left and right). All rats show consistent side preferences. Doses (0.5-2.0 mg/kg) of damphetamine differentially affected performance under signaled and nonsignaled conditions. Performance during the nonsignaled condition was much more sensitive to a drug-induced rate increment and timing impairment than performance during the signaled condition. With increasing drug dosage, under both conditions, side preferences reliably increased, decreased or remained unchanged depending upon the particular pattern of paw usage and the relationship between paw and side preferences. For the nonsignaled condition but not for the signaled condition, baseline rates were related to the strength of side preferences; lower rates and better timing performance wwere significantly correlated with greater preferences. Observations of bar-pressing behavior suggested that stereotyped motor patterns associated with side preferences might be related to mechanisms involved in timing behavior and perhaps, in behavior controlled by internal stimuli generally.