It was examined histomorphometrically whether or not and how chronic treatment with nasal salmon calcitonin (SCT) of rats could prevent ovariectomy (OVX)-induced osteoporotic changes in the trabecular bone of tibia. During 7 weeks on a synthetic low Ca diet after OVX, rats developed type 1 osteopenia which was defined by strut analysis as resulting mainly from loss in the connectivity of strut but in the thickness. An intermittent dosing regimen of nasal SCT (10 or 40 U/rat/d, 3 d/week, for 7 weeks) was able to retard development of osteopenia in a dose-dependent manner. The results indicate that the nasal route would be usable for chronic treatment of experimental osteoporosis with SCT and possibly other peptide anti-osteoporotics.