Fertilized mouse ova incorporated more (32p)-phosphate and (3H)-adenosine than (3H)-uridine or (3H)-guanosine into TCA insoluble material. The (3H)-guanosine-labelled product synthesized 1-3 hours after fertilization released labelled Gp and 7-methylguanosine-5'-phosphate on digestion with a mixture of ribonuclease A, T1 and T2 followed by nucleotide pyrophosphatase. Hydrolysis of (3H)-uridine-labelled material with alkali, or ribonucleases A, T1 and T2 produced Up as well as Cp. These results suggest that some RNA may be synthesized by the mouse egg 1-3 hours after fertilization. However, (32P)-phosphate-labelled mononucleotides were not detected in the alkaline or snake venom phosphodiesterase digest of (32P)-phosphate-labelled material, and most of the label was present as oligoadenylic acid following digestion of (3H)-adenosine-labelled material with ribonuclease A and T1. Unfertilized mouse ova also incorporated (3H)-adenosine and (32P)-phosphate into TCA insoluble material. Oligoadenylic acid was found in the ribonuclease A and T1 digest of (3H)-adenosine-labelled material and 2'-(5'-phosphoribosyl)-5'-AMP was identified in the snake venom digest. The later result suggests that ADP-ribosylation of proteins is occurring in the unfertilized ova. Primary amines were found to effect head-tail dissociation of mammalian sperm. This head-tail separation was prevented by reduction of the intact sperm with sodium cyanoborohydride suggesting that head-tail junction of mammalian sperm involves an aldoimine bond.