A series of 5H-thiazolo[3,2-a]pyrimidine derivatives 1 was studied with respect to the inhibition of 1S,3R-ACPD (10 mu M)-stimulated GTP gamma(35)S binding on rat mGlu2 receptor transfected cell membranes. The influence of substituents at position 6 and 7 as well as the substitution pattern of the two phenyl-rings in position 2 and 5 on the activity is discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.