Renal failure causes various metabolic disorders, especially increases of uremic toxins. Of the uremic toxins, guanidino compounds have a main role in uremia. Although lung injury is often complicated by renal failure, the role of guanidino compounds in lung injury is obscure. I have studied the changes in guanidino compounds in the lung of rat with ischemic acute renal failure. Guanidino compounds increased the first day after ischemia, but methylguanidine, which is the most toxic guanidino compound, showed a delayed increase in the lung. These results suggest the de novo synthesis of methylguanidine in the lung. A guanidinogram may by useful to judge changes in guanidino compounds.