Reparametrization invariance of string amplitudes is closely related t
o the non-polynomial closed string field theory. We show that it is po
ssible to formulate the terms of the non-polynomial action just by usi
ng the SL(2, C) properties of the so-called ''restricted vertices''. I
n this context we work out in detail the four point amplitude in the d
ifferent channels in terms of the reparametrization parameters.