Tissue contents and plasma levels of IR-CGRP were studied following administration of capsaicin to newborn rats. A depletion of > 50% of IR-CGRP content was seen in the cardiovascular tissues (e.g., heart and peripheral arteries), lungs, gastrointestinal tract (e.g., esophagus, stomach, and intestine), genitourinary tract (e.g., ureter, bladder, uterus, and penis), and in the nervous system (e.g., dorsal root and trigeminal ganglia, sciatic and trigeminal nerves, and dorsal spinal cord) in capsaicin-treated rats, in comparison with the control rat tissues (p < 0.01). These findings are compatible with the known involvement of capsaicin of the unmyelinated sensory C and A delta fibers and hence their distribution in the nervous system and other organs. Plasma IR-CGRP levels were also significantly lower in the capsaicin-treated rats throughout their life span (p < 0.001), suggesting that, at least in part, circulating CGRP is derived from the nervous system. RP-HPLC confirmed the identity of CGRP in both tissue and plasma extracts.