Prostaglandin E and F were measured in the testis, epididymis and vas deferens of six strains of inbred mice (A/J, AKR/J, C57BL/6J, DBA/2J, BALB/J and CE/J. The concentration of PGE and PGF was significantly higher in the vas deferens than in either the testis or the epididymis. Furthermore, the cauda epididymis contained a higher level of PGs than the caput portion. The relative preponderance of PGF to E varied between the tissues. The levels of PGs in the studies tissues indicated that they might be directly associated with the biological process of sperm maturation. The significant differences among strains of mice suggested that the observed variation might be, to a large extent, genetic.