The effect of synthetic salmon calcitonin (CT) on calcium- and pentagastrin-stimulated plasma CT was investigated in male Sprague-Dawley rats. Plasma samples were obtained before and 3 and 6 min after calcium (1 mg/100 g BW, iv) or pentagastrin (5 µg/100 g BW, iv) infusion. Although salmon CT at the dose of 0.4 MRC U/100 g BW administered 15 min before either the calcium or pentagastrin infusion did not influence circulating endogenous CT levels, salmon CT in doses of 4 and 20 U/100 g BW administered 15 min before the injection of secretogogues significantly reduced the response of CT to either calcium or pentagastrin without significantly changing plasma calcium levels from those of control animals. Salmon CT (20 U/100 g BW) given 5 min before pentagastrin infusion again significantly suppressed the response of plasma CT to this stimulus. Similarly, salmon CT administered chronically in a dose of 2 U/100 g BW for 28 days significantly reduced the response of CT to the calcium infusion. These data suggest that in the rat, at least pharmacological dosage of CT modulates the production and/or secretion of glandular CT itself.