This study was designed to assess the role of postovulatory ovarian and adrenal progesterone secretion in the regulation of ovum transport in pregnant rats. The number and distribution of embryos in the genital tract and the plasma progesterone levels were determined during the first 5 days of pregnancy in ovariectomized (OVX), adrenalectomized (ADX), ovariectomized and adrenalectomized (OVX + ADX), intact control (IC), or sham-operated control (SOC) rats. Some OVX or OVX + ADX rats received subdermal implants of progesterone pellets following surgery. The day of insemination was designated Day 1 of pregnancy. Transport of embryos from the oviduct to the uterus was essentially complete by the morning of Day 5 in IC and SOC. Neither adrenalectomy performed on Day 1, nor ovariectomy done on Day 4 of pregnancy modified the time of passage of ova from the oviduct to the uterus. Retention of some eggs in the oviduct, accompanied by rapid transport of others into the uterus and expulsion through the vagina, was observed when ovariectomy was performed on Days 1, 2 or 3, or when ovariectomy plus adrenalectomy were performed on Day 1. Following ovariectomy plus adrenalectomy or ovariectomy but not adrenalectomy alone, there was a marked and sustained decline in serum progesterone concentration. Subdermal implants of progesterone supplied on Day 1 restored progesterone levels and improved the pattern of ovum transport in OVX or OVX + ADX. It is concluded that postcoital secretion of progesterone by the ovary plays a major role in the physiologic regulation of oviductal embryo transport in the rat. Progesterone secreted by the adrenal is neither essential nor can it substitute for the ovarian source in the regulation of this process.