The rheological behaviour of boric oxide and alkali borate glasses has been studied in the region of high viscosity (1010–1013 poise). The change in viscosity of boric oxide glass with the introduction of alkali oxide is in agreement with what is known about the structure of alkali borate glasses. The rheological behaviour of boric oxide glass appeared to be of the Newtonian type up to a viscosity of 1013 poise, whereas the alkali borate glasses showed a Bingham flow behaviour at viscosities higher than 1011 poise. The experiments have yielded evidence of the correctness of the theory of Stein et al. concerning the rheological behaviour of glasses. A possible correlation between the phenomena of Bingham yield value and phase separation was not observed.