The grafting or adsorption of poly(ethylene glycol) based materials onto the surface of colloidal carrier systems can reduce the uptake of such particles by the reticuloendothelial system (RES) after intravenous injection. One of the most successful coating materials is the block copolymer Poloxamine 908, which has been shown to considerably decrease the liver uptake and extend the circulation half-life of polystyrene or poly(lactide-co-glycolide) (PLGA) nanospheres. Avoidance of the RES allows the particles the opportunity of access to specific sites such as tumours or areas of inflammation. The capillary endothelia at these sites are known to be leaky and will allow extravasation of the particles. Hence the delivery of drugs encapsulated in these particles would be advantageous.A model drug, Rose Bengal, has been incorporated into biodegradable PLGA nanospheres coated with Poloxamine 908 to produce slightly negatively charged nanospheres of 160nm in diameter with a hydrophilic coating layer.The distribution of radiolabelled drug in the coated nanospheres was followed after intravenous injection into rats and rabbits. Drug delivery in 908 coated nanospheres greatly extended the blood half-life compared with free drug. The organ distribution was also influenced, including increased drug delivery to the femur in the rabbit model at 24 hours post-injection.In addition, the fate of the drug was followed by gamma scintigraphy in the rabbit model, which supported the evidence for altered biodistribution. Free drug rapidly accumulated in the liver region and was then excreted through the gut. Delivery in nanospheres clearly extended the circulation of the model drug since the outline of the rabbit was visible up to several hours after injection. Also, the gut accumulation was considerably delayed.