19F nuclear magnetic resonance imaging (MRI) can be used as a non-invasive tool to simultaneously determine the location, the integrity and the oxygen supply of Ba2+-alginate implants. This requires that the beads (implants) are pre-loaded with the perfluorocarbon compound F-44E. Implantation of solid 19F-labelled beads into the peritoneum, below the kidney capsule or into the muscle of Wistar WU rats demonstrated that these beads could be detected by 19F-MRI for up to 18 months after implantation. This indicated that F-44E is not considerably released from the beads during implantation. The signal to noise ratio of liquid-core beads was higher by a factor of 4 than the signal to noise ratio of solid beads, but liquid-core beads were more fragile and also too large for implantation under the kidney capsule and into the intramuscular tissue. Quantitative 2-dimensional 19F-T1 maps (resolution 0.5 x 0.5 mm) could be deduced from 19F-MRI measurements. These T1-maps correlated to the local pO2-values. The partial oxygen pressure estimated in F-44E-loaded Ba2+-alginate beads showed that the oxygen supply inside the beads was very poor when they were implanted below the kidney capsule or into the peritoneal cavity. These low pO2-values obtained for the renal subcapsular site and the peritoneum may explain the failure of previous immunoisolated islet transplantation studies using these locations.