  We investigate the behavior of disclination loops in the vicinity of the
first order nematic-isotropic transition in the Lebwohl-Lasher and related
models. We find that two independent measures of the transition temperature,
the free energy and the distribution of disclination line segments, give
essentially identical values. We also calculate the distribution function D(p)
of disclination loops of perimeter p and fit it to a quasiexponential form.
Below the transition, D(p) falls off exponentially, while in the neighborhood
of the transition it decays with a power law exponent approximately equal to
2.5, consistent with a "blowout" of loops at the transition. In a modified
Lebwohl-Lasher model with a strongly first-order transition we are able to
measure a jump in the disclination line tension at the transition, which is too
small to be measured in the Lebwohl-Lasher model. We also measure the monopole
charge of the disclination loops and find that in both the original and
modified Lebwohl-Lasher models, there are large loops which carry monopole
charge, while smaller isolated loops do not. Overall the nature of the
topological defects in both models is very similar.
