This paper studies calcitonin (CT) transport across the C-cell-blood (CCB) barrier in normal rats. The efficiency of this transport is expressed as endogenous calcitonin bioavailability (F(CT)), i.e., that portion of CT molecules extruded by the thyroid C-cell basal pole that enters the circulating blood. Because currently F(CT) cannot be estimated directly, a novel methodology was used for its estimation. The essence of this methodology is to assess the CT level before and after the CCB barrier, to study these input-output CT relationships mathematically, and to find the F(CT) range allowed by the relationships. In this context, the subplasmalemmal granule numerical density in the C-cell basal pole (X) and the CT concentration in the right ventricular blood (Y) were found to be highly correlated: r(xy) = 0.922, P < 0.001; the covariance coefficient rho(xy) = 0.9980, P < 0.0001. Clearly, the overall variance of these input-output relationships in the covariance model (square root Var(xy) = 6.32) is explained, specifically, by CT retention (R(CT)) variability at the CCB transport stages. The greater the R(CT) mean value in the studied rats, the greater is the impact of R(CT) variability on Var(xy). The estimated variance caused by R(CT) (Var(CCBR)) makes it possible to determine R(CT) and F(CT) (F(CT) = 100% - R(CT)). Thus we estimated Var(CCBR) as Var(CCBR) < Var(xy) - CV(RIA)(2), where CV(RIA)(2) is CT assay variance: square root Var(CCBR) = 6.00% (P > or = 0.95). We then estimated F(CT) as the value range that does not contradict the Var(CCBR) obtained: 62% < F(CT) < 100% (P > or = 0.95).