The rheological behavior of a thermotropic main-chain random copolyester containing flexible spacers of 6 and 12 methylene groups (PSHQ6-12) was investigated. This copolyester undergoes only two thermal transitions : (i) glass transition at ca. 92 degrees C and (ii) nematic-isotropic transition at ca. 192 degrees C. Such a very broad range of temperatures between the glass transition temperature and the nematic-isotropic transition temperature allowed us to take rheological measurements in the transient/steady-state shear flow mode or in the oscillatory shear flow mode over a wide range of temperatures in the nematic region. The initial conditions (i.e., the initial morphology) for transient shear flow were controlled by first heating an as-cast specimen to the isotropic region, shearing there at a rate of 0.1 s(-1) for ca. 10 min, and then cooling slowly down to a preset temperature (140, 150, 160, 170, or 180 OC) in the nematic region. A fresh specimen was used for each transient shear flow experiment, and the shear stress growth (sigma(+)(t,y)) and first normal stress difference growth (N-1(+)(t,y)) were recorded as functions of time (t) for various shear rates (y) and temperatures. In the nematic region, PSHQ6-12 exhibited a single overshoot in sigma(+)(t,y), but multiple overshoots in N-1(+)(t,y) with its peak value greater than that of sigma(+)(t,y) In the isotropic region, however, PSHQ6-12 exhibited Newtonian behavior with negligible overshoot in both sigma(+)(t,y) and N-1(+)(t,y). Only positive values of the steady-state first normal stress difference (N-1) were observed in PSHQ6-12 for all temperatures and sheat rates investigated. Also conducted were intermittent shear flow experiments after cessation of start-up sheat flow, shouing evidence of structural recovery in PSHQ6-12 as determined by the overshoot in N-1(+)(t,y). The steady-state shear viscosity of PSHQ6-12 exhibited three regions at 140 and 150 degrees C : (i) a shear-thinning region at very low y, (ii) a Newtonian region at an intermediate range of y, and (iii) another shear-thinning region at higher y. But PSHQ6-12 exhibited a very mild shear-thinning behavior as the temperature was increased to 180 degrees C, approaching the isotropic region. Logarithmic plots of N-1 vs shear stress (sigma) for PSHQ6-12 were found to vary with temperature in the nematic region but to be independent oftemperature in the isotropic region. A similar temperature dependence was also observed in logarithmic plots of the dynamic storage modulus (G’) vs dynamic loss modulus (G "). We conclude that plots oflog N-1 vs log sigma or plots of log G’ vs log G " may be used to determine the isotropization temperature of thermotropic liquid-crystalline polymers.