  We calculate the thermal evolution of $\pi-\pi$ scattering lengths, in the
frame of the Nambu--Jona-Lasinio model. The thermal corrections were calculated
at the one loop level using Thermofield Dynamics. We present also results for
the pion thermal mass. Our procedure implies the modeling of a propagating
scalar meson as a resumation of chains of quark bubbles, which is presented
explicitly. We compare our results with previous analysis of this problem in
the frame of different theoretical approaches.
