This invention concerns heterocyclic derivatives which are useful in inhibiting oxido-squalene cyclase, processes for their preparation and pharmaceutical compositions containing them. The present invention is also concerned with heterocyclic derivatives capable of inhibiting cholesterol biosynthesis and hence in lowering cholesterol levels in blood plasma. The present invention also relates to methods of using such heterocyclic derivatives in treating or preventing diseases and medical conditions such as hypercholesterolemia, atherosclerosis and other medical conditions associated with elevated cholesterol levels. In formula (I): G is selected from CH or N; R1 is selected from hydrogen, halogeno, (1-6C)alkyl, halogeno(1-6C)alkyl, cyano, nitro, (1-6C)alkoxycarbonyl, and NR3R4 wherein R3 and R4 are independently selected from hydrogen and (1-6C)alkyl, and wherein up to 3 R1 groups may be present; T1 is selected from CH or N; T2 and T2 are independently selected from N and CR, wherein R is selected from hydrogen, hydroxyl and (C1-4)alkyl and wherein either ring containing T2 or T3 is optionally substituted with an oxo group; R2 is selected from hydrogen or (1-4C)alkyl; Q is selected from SO2, CO and CH2; AR is selected from a five or six-membered heterocycle containing up to 3 heteroatoms selected from nitrogen, oxygen and sulphur, phenyl, phenyl (2-6)alkenyl and naphthyl in which any Ar group is optionally substituted by one or more substituents selected from (1-6C)alkyl, halogeno, halogeno (1-6C)alkyl, (1-6C)alkoxy, (1-6C)alkoxycarbonyl, cyano, (1-6C)alkylamido, nitro, NR3R4 wherein R3 and R4 are independently selected from hydrogen and (1-4C)alkyl; provided that both T2 and T3 are not N and that when T2 is CR then T1 is not CH.