There is a great deal of controversy on the existence of NGF in body fluids and tissues. To date it remains unknown whether this peptide accumulates preferentially at significant levels in different organs. Thus we undertook the evaluation of kinetic parameters of the disappearance of blood of 125I-7S-NGF and 125I-beta-NGF after intravenous injection in male adult rats. Our results indicate that the plasma half-life of 125I-7S-NGF is approximately twice as long as for 125I-beta-NGF (respectively 61.7 +/- 11.7 min and 36.3 +/- 2.2 min) while the distribution volume is not significantly different between both peptides. Furthermore, the uptake of radioactive NGF by different tissues seems very low as shown by 125I-7S-NGF and 125I-beta-NGF content of the sampled organs compared to the plasma concentration at the same time. These results indicate that the tissue uptake of circulating 7S and beta-NGF is very low in the adult rat. Thus in these animals NGF did not cross the blood-brain barrier and did not accumulate in peripheral organs which are known to contain subsequent amounts of this peptide. This lack of deposition might be due to a binding with plasma proteins (probably alpha 2-macroglobulin).