Variation in canine gestation length was studied in a Beagle colony (n = 290) in which apparent gestation length, estimated as the interval from the day of first mating to the day of parturition, ranged from 57 to 72 days, and averaged 65.3 +/- 0.2 days. The interval from the day of the peak in luteinizing hormone (LH) to parturition was less variable and ranged from 64 to 66 days and averaged 65.1 +/- 0.1 days (n = 54). Apparent gestation lengths less than or equal to 61 days all resulted from mating greater than or equal to 3 days after the LH peak; those greater than or equal to 68 days all followed initial or single matings occurring greater than or equal to 2 days before the LH peak. Fertile single matings 3 days before the LH peak provided evidence that the potential postcoital fertile longevity of canine sperm is at least 6 days and thus contributed, along with variability in the onset of estrus, to the observed variation in apparent gestation length in the dog. The limited range in the interval from the day of the preovulatory LH peak to the day of parturition (64, 65, or 66 days) demonstrates a considerable regularity in the sequential events of gestation in the dog.