Synthetic procedures are described for the preparation of a variety of pyrimidine acyclo- nucleoside analogues, in which the aglycones are 5- and 5,6-substituted uracils, and the ribose moiety is replaced by different acyclic chains. These were examined as potential inhibitors of purified E. coli uridine phosphorylase. None of the compounds was a substrate for uridine phosphorylase, or either a substrate or inhibitor of E. coli thymidine phosphorylase. Kinetic measurements were employed to determine inhibition constants, K 1 , for inhibition of uridine phosphorylase. One of the more effective of these was 1-(1′3′-dihydroxy-2′-propoxy)methyl-5,6- tetramethyleneuracil, with K 1 = 2.7 μᴍ. The same compound was a reasonably good inhibitor of the reverse, synthetic, reaction, with K 1 values of 19 μᴍ vs uracil as the variable substrate, and 15 μᴍ νs α-ᴅ-ribose-1-phosphate as the variable substrate. For one of the analogues, which was a racemate, 1-(2′,3′-dihydroxypropyl)-5,6-tetramethyleneuracil, it was shown that only one of the enantiomers (R) was an inhibitor, the (S) enantiomer being totally inactive. For several of the analogues, the corresponding isomeric N(3)-acyclonucleosides were inactive as inhibitors. The results for several of the good inhibitors were compared with those of other observers for inhibition of uridine phosphorylase from mammalian sources. Preliminary measurements with several of our analogues demonstrated that some of them were indeed one to two orders of magnitude more effective against the enzyme from mammalian sources.