Somatostatin-14 (S-14) and somatostatin-28 (S-28) are generated by differential processing of a single precursor at a dibasic (R-K) or monobasic (R) proteolytic cleavage site, respectively. To study the pathways of processing of prosomatostatin, we have expressed in AtT20 cells cDNA encoding human prosomatostatin and prosomatostatin mutated in one or the other processing site. Analysis of the peptides present in cell extracts or culture media before and after stimulation of the cells with 8-BrcAMP indicated that prosomatostatin can enter three distinct secretory pathways where it is differently processed: 1) prosomatostatin was secreted through the constitutive pathway; 2) the regulated secretory pathway generated S-14 which was released upon stimulation of the cells with 8-BrcAMP; 3) an alternative pathway, insensitive to 8-BrcAMP produced S-28 and S-14. Moreover, our results suggest that the R-K processing site used to produce S-14 is an important structural feature for targeting the precursor to the regulated secretory pathway.