Bovine coronary smooth muscle cells (cSMC) synthesized the basic fibroblast growth factor (bFGF) and distributed it to two compartments. Resting cultured cSMC retained about 80% of total bFGF intracellulary, 20% is found in cell-associated (trypsin releasable) form as protease resistant bFGF-heparan sulfate complex. In contrast during exponential growth 20-30% of total bFGF is detected intracellulary, 70-80% is found at the cell surface. Both the cell-associated (pericellular) and intracellular bFGF are quantified by a highly specific immunoassay system. The endogenous bFGF was capable to stimulate cSMC replication and production of sulfated glycosaminoglycans to the same extent as reference human recombinant bFGF. Under culture conditions no bFGF was detectable in the culture medium. Heparin and low anticoagulant heparin inhibited the proliferation of cSMC in a dose-dependent manner up to 70% as compared with control cells. Interestingly heparin and low anticoagulant heparin caused an unexpected increase of the total bFGF content of cSMC by more than 60 %, especially as the cell-associated bFGF is concerned. At the same time the heparin treated cells are characterized by a significantly increased amount of FGF receptor (FGF-R1) and the cell membrane-integrated proteoheparan sulfate content.The results indicate a) that no correlations exist between the bFGF content and the tendency to replicate and b) that heparin prevents the formation of a trimolecular bFGF-receptor-HSPG complex which supports mitogenesis although all components of the complex are present at increased concentration.