Abstract: The present invention provides certain 6-hetarylalkyloxy pyrimidine derivatives of formula II ##STR1## wherein R is (i) a cyclic group having at least one 5- or 6-membered heterocyclic ring, optionally with a carbocyclic or heterocyclic ring fused thereto, the or each heterocyclic ring having at least one hetero atom chosen from O, N, or S, or a substituted derivative thereof; or (ii) phenyl or a substituted derivative thereof,R.sup.2 is selected from H, C.sub.1 -C.sub.5 alkyl, halogen or NH.sub.2,R.sup.4 and R.sup.5 which are the same or different are selected from H, NH.sub.2 or NO.sub.n where n=1 or 2, or R.sup.4 and R.sup.5 together with the pyrimidine ring form a 5- or 6-membered ring structure containing one or more heterocyclic atoms, and pharmaceutically acceptable salts thereof, exhibit the ability to deplete O.sup.6 -alkylguanine-DNA alkyltransferase (ATase) activity.