In vivo ESR studies have been made on Wistar and Sprague-Dawley rats using travelling wave helices implanted in the left lobe of the liver. Tissue regenerates completely around the Teflon-enclosed turns of the coil and histological studies have shown that it is composed of liver cells and connective tissue. Electron spin resonance studies of this tissue in vitro reported by Commoner and Ternberg in 1961 have demonstrated that it exhibits the 'tissue radical' signal. The implanted coils have detected, in vivo, a three-line spectrum due to a strong concentration of 4-hydroxy- 2,2,6,6-tetramethylpiperidine-1-oxyl free radical ('tempol'), a nitroxide spin label, injected either intramuscularly, intraperitoneally or intravenously into the experimental animal immediately prior to the measurement. They have also detected, in vivo, a similar three-line spectrum due to 'spin-labelled chlorpromazine', a tranquillizing drug to which this nitroxide spin label has been attached, injected intramuscularly or intraperitoneally into the animal immediately prior o the measurement. Work is currently in progress to increase the in vivo sensitivity of the implanted helices by menas of improved coupling techniques and the use of a time-averaging computer.