The effect of food deprivation on the rate of pregnancy failure in newly inseminated mice was evaluated. The pre-implantation fasting for 48 h beginning on day 4 or 5 postcoitum induced a high rate of pregnancy failure in newly inseminated females. However, fasting for 48 h on day 6 had a moderate effect in inducing pregnancy failure. By contrast, females which were deprived of food on day 7 postcoitum exhibited a significant reduction in the rate of pregnancy failure. Pre-implantation fasting was highly effective in inducing pregnancy failure compared to that of post-implantation fasting. The results suggest that fasting (i.e. day 4 or 5 postcoitum)--induced depression in prolactin has resulted in failure of pregnancy and fasting at post-implantation period (i.e. day 6 or after postcoitum) has not been interfered with progesterone production by placenta.