The influence of positions 11 and 24 on hypocalcemic potency and duration of action was examined [in rats]. These positions are, respectively, occupied by threonine and glutamine in HCT [human calcitonin], but by the basic amino acids lysine and arginine in SCT [salmon calcitonin and isohormones]. Replacement of threonine by lysine trebled the hypocalcemic potency of HCT and slightly prolonged its duration of action. Substitution of arginine for glutamine reduced the activity to about 1/10. Simultaneous introduction of both basic amino acids yielded an analogue intermediate in activity between those obtained by the single substitutions. The analogue [Bmp1 [.beta.-mercaptopropionic acid] Val8, Lys11, Arg24]-HCT displayed the same effects as [Lys11]-HCT. [Bovine, porcine, ovine and eel calcitonin were used].