A composition and method is provided to modify sperm fertility and sex ratio of mammalian offspring. The composition includes an amount of a class of compounds of phenoxazine or phenothiazine, having the structure (see FIG. 1) and ionic derivatives thereof, wherein R1 is H, a lower alkyl group or N(R3)2, R2 is N, S, O, and R3 is H or a lower alkyl group. A second component of the composition is an amount of an electron transfer agent capable of oxidizing a reduced cofactor (NADPH or NADH). A third component of the composition is an amount of a hexose sugar and/or the phosphorylated esters of such. When mixed with living sperm prior to or at the time of insemination, the composition results in modified rates of conception and an alteration of birth sex ratio in mammal species.