Serine protease inhibitors form a diverse family of proteins of which most members inhibit target serine proteases. Neuroserpin is a member of this family. Here, we have characterized neuroserpin in the nonmammalian species Xenopus laevis and found a high degree of aminoacid sequence conservation, especially of the reactive center loop, of the Xenopus protein compared to mammalian and chicken neuroserpin sequences, suggesting a conserved target specificity. Neuroserpin mRNA and protein were expressed throughout Xenopus development, while in the adult frog high mRNA expression was found in neuronal and neuroendocrine tissues, and the reproductive organs, and the neuroserpin protein was detected mainly in brain and pituitary. More specifically, in Xenopus pituitary neuroserpin mRNA was expressed higher in the neurointermediate lobe than in the pars distalis. At the protein level, we detected a 55-kDa neuroserpin protein in the pars nervosa, two neuroserpin proteins of 44- and 50-kDa in the melanotrope cells of the pars intermedia, and a 46-kDa product in the pars distalis. On the basis of its relatively high degree of sequence conservation and its expression pattern, we conclude that Xenopus neuroserpin may play an important physiological role, e.g. as a serine protease inhibitor during development, and for proper neuronal and neuroendocrine cell functioning.