Fluorine-19-nuclear magnetic resonance (19F-NMR) spectroscopic detection of the NMR-active Ca2+ indicator 5-fluoro-1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (5F-BAPTA) is one method for measuring cytosolic free Ca2+ concentration ([Ca2+]i) and has been used previously to measure [Ca2+]i in isolated cells and perfused organs. The aim of the present investigation was to demonstrate the feasibility of determining [Ca2+]i in vivo and in situ using 19F-NMR and 5F-BAPTA. Experiments were performed on male Sprague-Dawley rats with a surface-coil antenna employed for NMR interrogation. The Ca2+ indicator, 5F-BAPTA, was infused either intravenously (kidney, spleen) or intraventricularly (brain) as a 100 mg/ml solution of the cell-permeant acetoxymethyl ester (5F-BAPTA-AM) in dimethyl sulfoxide. Rats tolerated intravenous infusion without evident change in mean arterial blood pressure. In all tissues examined, kidney, spleen, and brain, [Ca2+]i was approximately 200 nM. To our knowledge, these results represent the first in vivo and in situ determinations of [Ca2+]i employing 19F-NMR.