A hydrodynamic theory describing the stalk dynamics and lipid flows upon BLM hemifusion was developed. The value of intermonolayer viscosity, etar, for membranes formed from azolectin mixed with lysophosphatidylcholine (7.10(-4) mg/ml) in n-decane, etar approximately 10(-9) g/s, was determined from comparison of the theoretical calculations and literature data. For membranes formed in squalene, the values etar approximately 10(-7) g/s for phosphatidylethanolamine and etar approximately 2-10(-7) g/s for azolectin were obtained. The calculated values are close to the published results of independent experiments which shows that the developed theory describes well the stalk growth and lipid flow.