Pubertal heifers can be classified between those with high (n=25) or low (n=15) antral follicle counts (AFC). The objective of this study was to determine oocyte development and maturation (e.g. fertility) in an IVF system for high- and low-AFC heifers. From a pool of 120 heifers, 10 high- and 10 low-AFC heifers were determined by transrectal ultrasonography; all heifers with evidence of oestrous cyclicity (i.e. pubertal) were synchronized with two 5-mL injections of prostaglandin F2 11 days apart. Heifers were euthanized over 4 days on Days 15 to 16 of the synchronized oestrous cycle. A total of 15 heifers (n=7 high and n=8 low AFC) were at the appropriate stage of the oestrous cycle. Ovaries were collected and transported to the laboratory. Follicles less than 8mm in diameter were aspirated. The IVF procedures and media were as previously described (Miles et al. 2004. Biol. Reprod. 71, 19191926). Cumulus-oocyte complexes (COC) were identified and washed in oocyte collection medium and then in maturation medium and were cultured (5% CO2; 38.5C) for 24h. Following maturation, COC were transferred and washed in fertilization medium. Thawed frozen semen from a crossbred bull was subjected to the swim-up procedure. Motile spermatozoa were collected and added to COC to yield a final concentration of spermatozoa per milliliter of fertilization medium. About 24h later, presumptive zygotes were washed in development medium, placed in microdrops of development medium, and cultured for 8 days. On Days 3 and 8 after fertilization, cleavage and blastocyst development, respectively, were assessed. Data were analysed using the Proc Mixed procedure of SAS (SAS Institute Inc., Cary, NC, USA) and the model included the effects of day of collection (n=4), group (n=7 high- or n=8 low-AFC heifers), and the interaction. The interaction did not differ (P=0.10). Day of collection influenced (P<0.05) the number of COC and the number of oocytes cleaved. High- compared to low-AFC heifers had the greater (P<0.05) numbers of COC (42.74.66 v. 22.14.59), oocytes that cleaved (28.13.60 v. 15.93.55), and developed to blastocysts (13.21.71 v. 6.21.69). However, there was no difference (P>0.10) in the percentage of COC that cleaved (65.35.58 v. 66.25.50%, high v. low, respectively) or that developed to blastocysts (46.76.75 v. 42.26.65%). In conclusion, AFC did not appear to affect oocyte maturation and development through the blastocyst stage.