The circadian clock involves co-operative transcription of circadian genes including Clock and regulates circadian rhythms in mammals such as the rhythm of sleep and wakefulness. In reproductive physiologies in mammals, there are several time-dependent events such as progression of the oestrous cycle and embryonic development, and we therefore checked the reproductive characters of mice homozygous for Clock delta-19 mutation (CL) having a diminished circadian clock to elucidate the effect of the circadian clock on reproductive physiologies. Profiles of the oestrous cycle were the same in female (f) wild-type mice (WT) and fCL: oestrous cycles of both fWT and fCL consisted of the same 3 stages, proestrous, oestrous, and metestrous/diestrous stages, and the average lengths of each stage and one oestrous cycle were the same in 10 fWT and 14 fCL. We therefore compared outcomes from the 4 possible mating groups between WT and CL. Average numbers of newborn pups obtained from mating pairs of 14 male (m) WT × fWT, 10 mCL × fWT, 14 mWT × fCL, and 15 mCL × fCL were 13.4 ± 0.8, 12.6 ± 0.4, 12.3 ± 0.7, and 8.6 ± 1.5, respectively, and gradually decreased depending on the number of mutated Clock alleles in mothers and embryos. Since increases in body weights of the mothers during the gestation period were not different in the 4 mating groups and since there were no signs of spontaneous abortion from mid to late gestation, we reasoned that some embryos were lost before or at the time of implantation. Immediately before implantation (88 h after fertilization), neither the number of embryos collected from uteri nor the percentage of embryos that reached the pre-implantation stage (blastocyst stage) differed significantly among mating groups. In contrast, immediately after implantation (160 h after fertilization), the average numbers of implantation sites in mating pairs of 12 mWT × fWT, 11 mCL × fWT, 13 mWT × fCL, and 13 fCL × fCL were 13.0 ± 1.5, 13.1 ± 1.2, 11.7 ± 0.8, and 7.0 ± 1.3, respectively, and also gradually decreased with increase in the number of mutated Clock alleles in mothers and embryos. This decrease was accompanied by a significant lowering of the positions of implantation sites in uteri, a possible cause of the decrease of the number of newborns and implantation sites, and average percentages of embryos implanted in a lower part of the uterus were 48.8, 59.3, 69.7, and 77.7% in mWT × fWT, mCL × fWT, mWT × fCL, and mCL × fCL, respectively. The difference between the mCL × fWT and mWT × fWT groups was statistically significant (P < 0.05), and this difference was thought to be due to the difference in embryonic genotype, specifically between WT and heterozygous embryos. However, the distribution of implantation sites in the mWT × fCL group was significantly smaller than that in the mCL × fWT group (P < 0.05), presumably due to the difference in maternal genotype, specifically between WT and homozygous mutant dams. This study showed involvement of the circadian clock, possibly the maternal and embryonic circadian clock, in implantation among events that occur from fertilization to parturition.