The effect of hrRANTES was studied after the injection in the sole of the rat paw, an area particularly rich in mast cells. Subcutaneous injections of RANTES 50 ng/10 microl produced an erythematous reaction which was inhibited by anti-RANTES antibody 50 microg/rat injected in the tail vein 30 min before hrRANTES 50 ng/10 microl was injected. In another set of experiments the animals were injected subcutaneously in the sole of the paw with PBS 10 microl (control), LPS (100 ng/10 microl) hrRANTES 50 ng/10 microl or anti-RANTES 50 microl/rat injected in the tail vein 30 min before hrRANTES 50 ng/10 microl was injected. The biopsies were analysed after 4 h and counted in an optic field. hrRANTES produced a strong recruitment of mast cells selectively coloured with 0.1% toluidine blue and inhibited by anti-RANTES antibody. In addition to the optical and electron microscope study, in some of the excised tissue Northern blot analysis for histidine decarboxylase (HDC) mRNA was performed to estimate the amount of histamine generation in the tissue of the injection sites. We found that subcutaneous injections of hrRANTES 50 ng/10 microl in the sole of the rat paw produced an accumulation of a great number of mast cells compared to PBS 10 microl (negative control) or LPS 100 ng/10 microl (positive control) after 4 h. The hrRANTES effect was inhibited by anti-RANTES antibody injected in the tail vein 30 min before hrRANTES exposure. Moreover, hrRANTES increased HDC mRNA and histamine generation.