Calcitonin gene related peptide (CGRP) plays an important role in the CNS and in the cardiovascular system. To identify high-affinity antagonists in competitive binding studies, we identified a novel radioactive tracer, [(3)H-propionyl-K(24)]-halphaCGRP 8-37, which was labeled in solution by a recently developed strategy using photolabile protecting groups at reactive side chains. This tracer was shown to be as potent as commercially available (125)I-tracers for the determination of agonists and to have increased sensitivity for antagonists. We applied it to investigate the predicted turn structures centered at Pro(29) and Pro(34). The substitution at positions 29 and 34 by turn-inducing amino acid mimetica showed that these turns are highly diverse. At position 29, a hydrophobic residue is preferred that constricts the secondary structure, whereas position 34 is required to stabilize the conformation of the backbone. All high-affinity analogues showed antagonistic properties with potency similar to CGRP 8-37.