Calcium transfer across the blood-nerve barrier of the frog sciatic nerve was studied using an in situ perfusion technique and an in vivo i.v. bolus injection technique. The permeability-surface area product of 45Ca at the blood-nerve barrier, (PA)BNB, calculated from radioactivity in the desheathed nerve segment after 5 min of circulation of tracer, and corrected for the residual radioactivity in the blood space, equaled 4.4 +/- 0.4 (S.E.M.) X 10(-5) ml.s-1.g-1 wet wt. The (PA)BNB of 45Ca was independent of [Ca2+] in the perfusion medium between 0.18 and 18 mM. The permeability-surface area products of 45Ca across the perineurium [(PA)per] also was measured by an in situ incubation technique, and equaled 1.45 +/- 0.41 X 10(-5) ml.s-1.g-1 wet wt. (n = 8). The half time (t 1/2) for nerve calcium to equilibrate with plasma calcium was calculated to be 60 min. The low, passive permeability to calcium of the blood-nerve barrier probably limits marked calcium concentration changes in nerve endoneurium following transient changes of plasma calcium, but should not alter steady-state responses.