  We propose a method to infer the initial spin period of pulsars residing in
composite supernova remnants. Such a remnant consists of both a plerionic and a
shell type component, corresponding respectively to the pulsar wind nebula
driven by the spindown luminosity of the central pulsar, and the blastwave
bounding the supernova remnant. Theoretical investigations including
hydrodynamical simulations have shown that at late times (~ 1,000 - 10,000
years), a simple scaling law connects the radius of the supernova shell to the
radius of the plerion. The energy content of the plerion and the total
mechanical energy of the supernova remnant enter into this scaling law. One can
use this scaling law to estimate the initial spin period of pulsars residing in
composite remnants. We discuss potential pitfalls of this method, including the
effect of a small remnant age and of strong radiative losses in the plerion.
