Cows of mixed beef breeds were randomly assigned to receive one of three treatments: 1) i.v. injection of hCG on d 6 of the estrous cycle (estrus = d 0) and slaughter on d 12 to 14, 2) i.v. injection of saline on d 6 and slaughter on d 12 to 14, or 3) slaughter on d 5 to 7. Corpora lutea (CL) were collected at slaughter and weighed. A portion of each CL was cultured for 6 h with no hormone or various doses of LH (25, 50, or 100 ng/mL of medium). All cows that received an hCG injection on d 6 (Treatment 1) exhibited a luteal structure in addition to the CL present at the time of hCG administration. Thus, the following CL were evaluated: 6-d-old (induced) CL from Treatment 1 (d-6 hCG) and 13-d-old (natural) CL from Treatment 1 (d-13 hCG), 13-d-old CL from Treatment 2 (d 13), and 6-d-old CL from Treatment 3 (d 6). From d 9 to 13, daily plasma progesterone concentrations were greater (P < .01) for cows on Treatment 1 than for cows on Treatment 2. Whereas LH stimulated (P < .01) progesterone secretion regardless of treatment, progesterone secretion in vitro was greater (P < .01) for CL from saline-treated cows (Treatments 2 and 3) than for CL from hCG-treated cows (Treatment 1). No effect of hCG was observed for luteal DNA, RNA, or protein concentrations, or for luteal RNA:DNA or protein:DNA ratios.(ABSTRACT TRUNCATED AT 250 WORDS)