  We study the sequences of numbers corresponding to lambda terms of given
sizes, where the size is this of lambda terms with de Bruijn indices in a very
natural model where all the operators have size 1. For plain lambda terms, the
sequence corresponds to two families of binary trees for which we exhibit
bijections. We study also the distribution of normal forms, head normal forms
and strongly normalizing terms. In particular we show that strongly normalizing
terms are of density 0 among plain terms.
