A highly selective melanocortin-4 receptor antagonist cyclic hexapeptide of the formula ##STR00001## where R.sub.1, R.sub.2, R.sub.3a, R.sub.3b, R.sub.4, R.sub.5, x, y and z are as defined in the specification, and a method of treating body weight disorders, including cachexia, sarcopenia and wasting syndrome or disease, and treating inflammation and immune disorders.