Ovarian and luteal blood flow rates were studied using radioactive microspheres in guinea-pigs between Day 6 of the oestrous cycle and Day 1 of the following cycle. Peripheral plasma progesterone levels were measured by radioimmunoassay on the same days of the oestrous cycle. Ovarian blood flow was greatest between Days 9 and 12 and had fallen by Day 16 both in absolute (ml . min-1) and relative (ml.min-1.g-1) terms. Luteal weight and blood flow were also greatest between Days 9 and 12 and had fallen sharply by Day 16. The highest mean (+/- s.d.) luteal flows measured were 0.10 +/- 0.04 ml.min-1 per corpus luteum, and 24.26 +/- 9.3 ml.min-1.g-1 luteal tissue on Day 10 of the cycle. Mean peripheral plasma progesterone levels reached a maximum of 3.66 +/- 1.1 ng/ml at Day 12 of the cycle and fell thereafter, reaching 0.74 +/- 0.5 ng/ml by Day 1 of the following cycle. Plasma progesterone levels declined significantly between Days 12 and 14 of the cycle, whereas no significant drop in luteal blood flow was demonstrable until after Day 14. These data do not support the idea that declining luteal blood flow is an initiating mechanism in luteal regression in the guinea-pig.