Dispersed cells from rat corpora lutea isolated on pregnancy days, 5, 7, 10 and 13, yielded three distinct sub-populations of luteal cells when centrifuged in a continuous, 0-40%, Metrizamide density gradient. Bands I and II (refract, ind. 1.340-1.346) constituted highly enriched, functional, luteal cell fractions. The more dense fraction (Band III, refract, ind. 1.348-1.355), while contaminated with non-luteal cells, also contained cells capable of progesterone production. Progesterone synthesis by all three sub-populations was stimulated by LH, and each sub-population bound [125I]-labeled hCG and [125I]-labeled HDL with high affinity. However, the concentrations of LH/hCG and lipoprotein binding sites were greater in Band I + II than in Band III. Progesterone secretion during the selected days of pregnancy, being high on days 5 and 13 with a nadir on day 10, was shown to correlate with the concentration of LH/hCG-binding sites present on Band I + II cells. This correlation was not seen when comparing Band III cells. The concentration of HDL-binding sites did not vary significantly with the sub-populations of luteal cells isolated on the selected days of pregnancy.