Two related molecules from the acyclic nucleoside family 9-(1,3-dihydroxy-2-propoxymethyl) guanine (DHPG) and 9-(1,5-dihydro-4-hydroxymethyl-3-oxapentyl-2- [R])guanine (2', 3'- secoG) have been compared by means of force field conformational analysis. They are respectively active and nonactive analogs of the antivirial compound 9-((2-hydroxyethoxy)methyl)guanine (ACG). As in the case of ACG many local minima are found for both molecules, indicating their great flexibility. For all three molecules conformations similar to those occurring in cyclic nucleosides have energies from 3 to 7 kcal mol−1 above the most stable minima.