A comparison of a closely related set of cyclic octapeptides demonstrates how Nature has adapted two common amino acid building blocks (Thr, Cys) as conformational ring constraints (oxazoline, thiazole) to regulate the three-dimensional structures and reactivities of marine macrocycles. A 2D NMR spectroscopic study shows that conversion of two Cys residues in the flexible cyclic octapeptide 1, c[Ile-Thr-D-Val-Cys-Ile-Thr-D-Val-Cys-], to 5-membered thiazole rings (Thz) leads to the formation of a novel pseudochair conformation in 2, c[Ile-Thr-D-(Val)Thz-Ile-Thr-D-(Val)Thz-]. The conformational flexibility of 2 is significantly restricted by three intramolecular hydrogen bonds induced by the D-(Val)Thz components, resulting in a single solution conformation with nan-C-2 symmetric side chains, Additional modification, through conversion of the two threonine side chains to 5-membered oxazoline rings (Oxn), produces a highly constrained pseudoboat or saddle-shaped macrocycle, c[(Ile)Oxn-D-(Val)Thz-(Ile)Oxn-D-(Val)Thz-] (7), having C-2 symmetric side chains. Acid hydrolysis of 7, previously isolated from the ascidian Lissoclinum patella, selectively opens the two oxazoline rings with further conformational rearrangement to a navel cyclic octapeptide (8) possessing a shallower pseudoboat conformation. The comparison reveals that oxazoline and thiazole rings impose seven conformational restrictions upon these cyclic octapeptides, creating unusual shapes and clefts with varying capacities to capture organic or metal ion guests (e.g. 10). Such dramatic changes in macrocycle shape may be related to the differential antitumour and metal-binding properties of this class of molecule.