This study was designed to describe the follicular population present on the canine ovary (Canis familiaris) during the preovulatory period and essentially the changes in oocyte size, mucification, and chromatin configuration occurring from before the luteinizing hormone (LH) surge up to postovulation. In a first experiment, ovaries of beagle bitches were collected before (n=21) or after LH surge but before ovulation (post-LH surge/preovulation stage, n=24) as determined using hormone (LH, estradiol, progesterone) assays and ultrasonography. All large (>2mm) follicles were measured and punctured. The numbers of oocytes collected per follicle and the degree of cumulus mucification were recorded. In a second experiment, ovaries were similarly collected before (n=13) and after the LH surge but before ovulation (n=11) as well as after ovulation as determined by ultrasonography (n=9). Chromatin configuration of the oocytes was observed by DNA staining and confocal microscopy. In Experiment 1, before the LH peak, an average of 13.5+/-0.7 follicles per bitch (total 284 follicles) were detected, and the maximal follicle diameter reached 6.5mm. Large follicles were observed already in this period of the cycle and as early as when progesterone was still below 0.5 ng/mL. After the LH peak but before ovulation, 11.0+/-0.7 follicles were present (total 264 follicles). Fully mucified cumulus cells were observed only in follicles larger than 4mm. Multi-oocytic follicles represented 7% (before LH peak) and 4% (after LH peak) of the follicular population. In Experiment 2, all the oocytes were at the germinal vesicle (GV) stage, but three chromatin configurations could be distinguished: diffuse, partly grouped, and fully grouped chromatin. The proportion of oocytes with fully grouped chromatin increased with the follicular diameter and the time in estrus, the maximum being observed after the LH peak. These results suggest that (1) before LH peak, follicles are already of large diameter, similar to the ones at ovulation; (2) the ability for cumulus mucification is acquired during the late steps of follicular growth; (3) three GV patterns may be observed during the periovulatory period.