Two methods for the collection of pituitary portal blood were compared. The portal blood aspiration method developed by Worthington and Fink consists of periodically aspirating portal blood as it collects around the transected pituitary stalk. The stalk cannulation method developed by Porter eliminates possible contamination of the sample by extraneous blood by collecting the portal blood into a polyethylene cannula which fits tightly around the stalk. In this study, contamination of portal blood collected by the portal blood aspiration method was identified and quantitated by measuring now rates and LH concentration of blood from the various sources contributing to the collection. Blood collected by the portal blood aspiration method contained 68% pure portal blood, 21% extraneous peripheral blood from damaged tissues, and 11% pituitary blood from the distal end of the transected stalk. The flow rate of pure portal blood was the same in the two methods. Despite the problem of contamination and dilution, the portal blood aspiration method is well suited for measurement of labile substances because the blood can be added to appropriate preservatives within seconds. For the measurement of more stable substances, the stalk cannulation method is preferred.