In this report we describe the development of a radioimmunoassay for neuropeptide K (NPK) that is not influenced by the presence of free neurokinin A (NKA) or neuropeptide gamma (NPG). The antisera for the radioimmunoassay were obtained by immunizing rabbits with beta-preprotachykinin A fragment 69-91. This fragment contains the sequence 1-20 of NPK and has only the sequence 18-20 (Gly-His-Gly) in common with NPG. Two antisera were obtained, neither cross-reacting with NKA or NPG. The specificity of these antisera was confirmed by the finding of a major immunoreactive peak in the eluates obtained from HPLC separation, corresponding to NPK. This radioimmunoassay was applied to the determination of NPK in hypothalamic and anterior pituitary extracts from Siberian hamsters. Hypothalami from male and female hamsters contained similar concentrations of NPK, although slightly higher in males. NPK levels in the hypothalamus of female Siberian hamsters showed few fluctuations during the estrous cycle, with the levels measured at estrus being significantly lower than at proestrus, diestrus I, and diestrus II. In anterior pituitaries from Siberian hamsters the concentrations of NPK were significantly lower in females than in males. This radioimmunoassay has demonstrated the presence of NPK in the anterior pituitary gland of the Siberian hamster, and it should be very useful for future studies on mechanisms of regulation of NPK secretion in different tissues.