The present invention relates to ethynyl derivatives of formula I wherein Y is N or C&mdash；R3； R3 is hydrogen, methyl, halogen or nitrile； R1 is phenyl or pyridinyl, each of which is optionally substituted by halogen, lower alkyl or lower alkoxy； R2/R2&prime； are each independently hydrogen, lower alkyl or lower alkyl substituted by halogen, or R2 and R2&prime； together with the N-atom to which they are attached form a morpholine ring, a piperidine ring or an azetidine ring, each of which is unsubstituted or substituted one or more substituents selected from lower alkoxy, halogen, hydroxy and methyl； R4/R4&prime； are each independently hydrogen or lower alkyl, or R4 and R4&prime； together form a C3-5 cycloalkyl-, tetrahydrofuran- or an oxetane-ring； or to a pharmaceutically acceptable acid addition salt, to a racemic mixture, or to its corresponding enantiomer and/or optical isomer and/or stereoisomer thereof. Compounds of formula I are positive allosteric modulators (PAM) of the metabotropic glutamate receptor subtype 5 (mGluR5).
