Abstract: Investigation of some tetrahydroisoquinoline alkaloids on their in vitro affinities to D-2,5-HT₁ and 5-HT₂ receptors in rat brain was made using radioreceptor binding assay. It was found that about 12 alkaloids of two different types (i. e. aporphines and protoberberines)influenced the binding properties of these three receptor sites to various extent. l-Stephanine (l-STP) exhibited relatively high affinity to D-2,5-HT₁ and 5-HT₂ receptors (K_i=1.7×10^-7mol, 9.4×10^-8mol and 1.8×10^-7 mol, respectively). The effect of l-roemerine (l-REM) on 5-HT₂ receptor appeared to be similar to that of l-STP, its K_i value was found to be 1.7×10^-7mol. The potencies of dl-tetrahydroberberine, dl-tetrahydrocoptisine and dl-tetrahydrojatrorrhizine on D-2 receptor are between those of l-stepholidine and l-tetrahydropalmatine. The majority of the alkaloids reported in this paper have been shown to bind to more than one receptor sites with varying affinities, indicating that they may possess complex interactions with monoamines and their transmission.