Possible local and systemic adverse effects following administration of salmon (sCT) and human (hCT) calcitonin (CT) have been evaluated in a double-blind, within-subject, comparative trial in 30 young, healthy volunteers. Each subject received 0.25 and 0.5 mg hCT and 100 IU sCT s.c.. Adverse effects and hypocalcaemia were recorded 1, 3 and 6 h after each injection. Significantly fewer local adverse reactions were observed after hCT (20 or 33%) than after sCT (80%), possibly due to the different vehicles employed (mannitol solution and acetic acid). The most frequent systemic adverse effects were gastrointestinal (nausea, vomiting), which occurred in 80% after 1 h, independently of the CT--preparation used. Hypocalcaemic changes were generally small and lasted longer after sCT. It is concluded that the hCT preparations were better tolerated locally than sCT in young, healthy volunteers, and that there were no differences in the systemic side effects or hypocalcaemic activity.