We obstructed renal lymph drainage from a single kidney in diabetic rats who had received 65 mg/kg streptozotocin 6 months prior to the study. Lymphatic obstruction led to a progressive fall in systematic blood pressure from a mean arterial pressure of 101±5 (SEM) mm Hg (n = 7) to 62±4 mm Hg (n = 5) (p<. 02) after 1.5 h. No change was seen in a sham-operated animal. Despite the decline in systemic blood pressure there was no significant change in the GFR of either kidney. Sodium excretion increased significantly in the experimental kidney. There was no change in the urinary excretion of cyclic GMP from either kidney, and plasma levels of atrial natriuretic peptide (ANP) did not change (55±21 pg/mL pre- to 64±18 postobstruction). The results are consistent with a systemic vasodilatation after lymphatic obstruction. The mechanism of this response is still under investigation, but apparently it does not involve ANP.