Puberty in the female cat occurs between the ages of 8 and 10 months. Cats are seasonally polyoestrous, reflex ovulators. The oestrous cycle can occur as often as every 2-3 weeks. Ovulation usually occurs 24-36 h after copulation, and implantation occurs 12-13 days after copulation. The duration of gestation in the cat is 64-67 days (average 66 days). The corpora lutea secrete increasing amounts of progesterone, starting 1-2 days after ovulation. If implantation occurs, progesterone concentrations continue to increase throughout days 25-30, then slowly decline throughout the rest of pregnancy. In the absence of pregnancy, the corpora lutea reach their peak progestational activity within 10-15 days and then decline, with basal progesterone values being noted by days 30-35. Relaxin is produced by the fetoplacental unit beginning at about day 20 of gestation and continuing throughout the rest of pregnancy. Prolactin production increases from about day 35. Like those of prostaglandin F2 alpha, concentrations of prolactin plateau at about day 50 and increase abruptly just before delivery.