We report measurements of the dynamics of the magnetic Frederiks transition in nematics consisting of disc-like molecules. In this paper the results are presented for three 2, 3, 6, 7, 10, 11-hexakis(p-alkoxybenzoyloxy)triphenylenes, which exhibit a normal nematic phase, and for three 2, 3, 7, 8, 12, 13-hexa(alkanoyloxy)truxenes, which exhibit an inverted nematic phase. We find that the thermal dependence of a bend viscosity coefficient (γ*1) can be accurately described by the expression, γ*1∼S2exp (Ea/kT). The absolute value of γ*1is found to be higher (by a factor of 10–100) than is commonly encountered in nematics consisting of rod-like molecules.