In Part 1, the presence and diameter of the corpus luteum was monitored daily by transrectal ultra-sonography for a minimum of 30 days in 39 llamas. Animals were categorized as non-lactating or lactating and randomly allotted to the following groups (reproductive status), (1) non-mated (anovulatory); (2) mated by a vasectomized male (ovulatory non-pregnant); (3) mated by an intact male (pregnant). In Part 2, luteal diameter was monitored by transrectal ultrasonography in 68 pregnant llamas to characterize its diameter profile for the first 60 days of pregnancy. Results indicated that measurement of the diameter of the corpus luteum by transrectal ultrasonography was an accurate method of assessing luteal function (plasma progesterone concentration) in llamas (r = 83%, P < 0.0001). Corpora lutea were not detected and the plasma progesterone concentration did not exceed 0.4 ng ml-1 in anovulatory (non-mated) llamas. The corpus luteum was first detected on mean Day 3.1 ± 0.2 for ovulatory non-pregnant and pregnant llamas, and reached the maximum diameter on mean Day 5.9 ± 0.3 and 21.4 ± 1.2, respectively (P < 0.0001); the maximum diameter was 12.8 ± 0.3 and 16.3 ± 0.3 mm, respectively (P < 0.0001). Except for the first day of detection of the corpus luteum, lactational status did not affect luteal growth and regression. Reproductive status (ovulatory non-pregnant vs. pregnant) did affect luteal growth and regression (P < 0.0001); the profiles were the same until Day 9 at which time the corpus luteum was regressing in non-pregnant llamas and still growing in pregnant llamas. The corpus luteum was not detected after Day 12 in non-pregnant llamas and was detected to Day 60 (end of observations) in pregnant llamas. A prolonged luteal phase (pseudopregnancy) was not observed in any non-pregnant (non-mated or vasectomy-mated) llamas. The plasma progesterone concentration fell (P < 0.05) on Day 8 in pregnant llamas, similar to non-pregnant llamas, but there was a resurgence of luteal function in pregnant animals after Day 10 (Day 6, 5.6 ng ml-1; Day 10, 3.1 ng ml-1; Day 12, 6.3 ng ml-1). Results demonstrated, (1) a close temporal relationship between ultrasonically determined luteal diameter and circulating progesterone concentration, except that progesterone decreased 1-3 days before the morphologic decrease; (2) regression of the corpus luteum between Days 9 and 11 in non-pregnant llamas versus a continued gradual increase until a plateau at Day 25 in pregnant llamas; (3) a fall in circulating progesterone in both non-pregnant and pregnant llamas on Day 8, but with rescue and resurgence of the corpus luteum in pregnant llamas after Day 10. The latter observation was unexpected and requires confirmation. © 1991.