The percent of cardiac output distributed to the rectal gland of 33 free-swimming fish was quantitated by the isotopically labeled microsphere technique. In 21 fish less than 1% was found in the rectal gland; 12 fish had 2-7%, suggesting a pattern of intermittent blood flow. Methylmethacrylate corrosion casts were made for scanning electron microscopy study of the microvasculature. Blood entering the rectal gland artery is distributed via three pathways. The most extensive is along the rectal gland artery into 11 or 12 circumferential vessels, which branch along the capsule and give rise to the radially arranged sinusoids that perfuse the secretory tubules and then join to form the central rectal gland vein. A second is an arterial bypass that runs the length of the rectal gland and joins the circulation of the postvalvular intestine. The third is via arteriovenous anastomoses in the capsule between tightly packed small arteries and veins. The low to moderate blood flow in most rectal glands and the very high flow in others is consistent with the observation that in vivo the rectal gland secretes only intermittently. Blood pathways allowing bypass of the rectal gland parenchyma suggest a role of blood flow in the regulation of the secretory process.