Chronic lithium treatment was examined for effects on the blood pressure of spontaneously hypertensive rats (SHRs). Treated rats were allowed continuous access to a pelleted lithium diet (1.7 g LiCl (40 mmol)/kg diet) for 21 days. Control SHRs were fed a similar diet lacking the lithium. Two groups of control rats were examined. One group had continuous access to control diet (ad lib controls); a second group was pair-fed a daily ration of control diet such that their mean body weight remained similar to the lithium treated group (pair-fed controls). Heart rate, systolic, diastolic and mean arterial blood pressures were recorded on day 21 from freely moving conscious rats. Systolic, diastolic and mean arterial blood pressure was significantly lower (approximately 15 mmHg) in the lithium treated SHRs as compared to either control group; the pair-fed and ad lib controls had similar blood pressures at the end of the treatment periods. Heart rate was increased in the lithium treated animals. All rats gained weight during the 21 days of treatment, although the lithium treated group and the pair-fed control group did not gain weight as much or as rapidly as the control group that had continuous access to control diet. Plasma and brain lithium levels were in the moderate range (0.3--0.4 meq/l) and all rats appeared healthy at the end of the experiment. These results suggest that chronic treatment may have clinically relevant effects on blood pressure.