More than 130 acyclic purine and pyrimidine nucleoside derivatives (I, R1 = OH, OCHMe2, or OCH2CF3, R2 = H, CHMe2 or Ac; and II, R2 = OH, H, NH2, or OCHMe2, R2 = OH, alkoxy, etc., R3 = H, OH, OAc, or OCHMe2) was synthesized.Structure-activity relations among a selected group of 6- and 9-substituted guanine derivatives were discussed.By introduction of secondary aliphatic (e.g. iso-Pr or secondary butyl) ether groups into acyclic nucleosides, e.g., acyclovir and gancyclovir, new lipophilic prodrugs with modified phys. and improved pharmacokinetic properties were obtained.Several compounds with isopropyl ether groups in the side-chain and/or in the 6-position (among these the 6-deoxy derivative HOE 602) of the purine moiety displayed excellent antiviral activity when tested in vivo against herpes simplex virus type 1 infection in mice.