INTERACTIONS of L-triiodothyronine (T3) in adult rat cerebrocortical synaptosomes were studiedin vitro.Scatchard plot analysis revealed two sets of T3binding sites. The degree of saturation of T3binding sites (putative receptor) correlated well with the dose-dependent inhibition of Na+-K+-ATPase activity in synaptosomes. The relative binding affinities and relative inhibition of enzyme activities for different TH analogues were L-T3> T3-amine > TRIAC = L-T4> r-T3> T2and L-T3> T3-amine > TRIAC > L-T4> r-T3> T2, respectively. The present study demonstrates the nature of inhibition of synaptosomal Na+-K+-ATPase activity may be as a function of T3occupancy of synaptosomal receptor sites in adult mammalian brain.