O.sup.6 -hetarylalkyl- or naphthylalkylguanine derivatives of the formula ##STR1## wherein Y is H, ribosyl, deoxyribosyl, or R"XCHR', wherein X is O or S, R" and R"' are alkyl, or substituted derivatives thereof, R' is H, or alkyl or hydroxyalkyl R is (i) a cyclic group having at least one 5- or 6-membered heterocyclic ring, optionally with a carbocylic or heterocyclic ring fused thereto, the or each heterocyclic ring having at least one hereto atom chosen from O, N, or S, or a substituted derivative thereof; or (ii) naphthyl or a substituted derivative thereof and pharmaceutically acceptable salts thereof, exhibit the ability to deplete O.sup.6 -alkylguanine-DNA alkyltransferase (ATase) activity. A process for preparation of the compounds is described. The compounds have utility in combination with alkylating agents in the chemotherapeutic treatment of tumour cells.