Mutt and Jorpes (49) originally isolated cholecystokinin (CCK) from porcine intestine. Subsequently, it was recognized that CCK/gastrin-like material could be found in the rat brain (74), and it was later shown mainly to represent the C-terminal octapeptide (CCK-8) (2, 12-14, 48, 54, 55). These radioimmunoassay studies have been supplemented by numerous immunohistochemical investigations showing extensive CCK immunoreactive neuron systems in the brain and spinal cord (20, 26, 27, 31, 37, 40, 42, 44, 72, 75, 76). During recent years several groups have employed in situ hybridisation and radioactively labelled probes complementary to CCK mRNA and partly confirmed results from immunohistochemical studies but also revealed new interesting findings (3, 5, 6, 30, 41, 58, 64-66, 77). Several lines of evidence indicate that CCK-8 may act as a neurotransmitter or neuromodulator in many areas of the central nervous system. The development of new CCK antagonists has opened up new possibilities to understand the functional significance of CCK peptides in the neurons and other systems. The aim of the present article is to briefly review the distribution of some of the CCK systems and in this way define possible targets for these new types of drugs. Focus will be on cerebral cortex in view of the theme of the meeting, anxiety, on spinal cord as a basis for discussion of CCK and pain, and finally CCK/gastrin peptides in sperm will be discussed. The question of coexistence of CCK and dopamine in mesencephalic neurons is reviewed in a parallel article (28).