When sodium nitroprusside in artificial medium was perfused through the isolated liver and hindlimbs of a rat at the near physiological flow rate of 8.5 ml min-1, free cyanide was found in the perfusate. The liver reached a steady-state ratio of cyanide released/nitroprusside perfused of about 1.5 (or approximately 30% of the total nitroprusside cyanide) within 15 min, and maintained that rate for about 1.5 hr. In the hindlimbs cyanide was released at a much slower rate (7.5 to 18.8% of the total), and the release did not achieve a steady state even after 1.5 hr. Even after small corrections for cyanide extraction by both tissues, the rate of cyanide release by either tissue was probably more rapid than that resulting from static incubations in blood.