In general, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) play important roles in the regulation of cumulus cell expansion and oocyte maturation. We investigated the effects of supplementation of FSH or LH in in vitro maturation (IVM) medium on the incidence of cumulus cell expansion and nuclear maturation in canine oocytes. Cumulus-oocyte complexes (COCs) were cultured in TCM-199 supplemented with 10% foetal bovine serum (FBS), 1 mg/ml cysteine, 0.2 mm pyruvic acid and different concentrations of FSH or LH (control, 0.5, 5 or 50 microg/ml) at 38.5 degrees C, 5% CO(2) in air for 72 h. The cumulus cell expansion was measured by microscopic visualization, and nuclear maturation of denuded oocytes was determined by staining with 10 microg/ml Hoechst33342 for 30 min. The cumulus cell expansion in the 5 microg/ml FSH group (397.2 +/- 64.3 microm) was significantly higher than those in the control, 0.5, and 50 microg/ml FSH groups (168.3 +/- 19.1, 286.0 +/- 69.7 and 300.0 +/- 84.3 microm, respectively; p < 0.05). However, there was no difference in cumulus cell expansion among the control, 0.5, 5 and 50 microg/ml LH groups (165.6 +/- 20.2, 160 +/- 26.5, 172 +/- 20.5 and 168 +/- 23.1 microm, respectively; p > 0.05). After 72 h of IVM, the proportion of nuclear development to the MI-MII stage in the 0.5 microg/ml FSH group (15.1%) was higher than those in the control, 0.5 and 50 microg/ml FSH groups (0.9%, 6.5% and 8.0%, respectively; p < 0.05). However, there was no significant difference in nuclear maturation to the MI-MII stage among control, 0.5, 5 and 50 microg/ml LH groups (4.6%, 2.3%, 5.4% and 8.6%, respectively; p > 0.05). This study indicated that a FSH supplement in IVM medium can increase cumulus cell expansion and nuclear maturation, while the nuclear maturation rate remained low. Further studies are required to improve the nuclear development to the MI-MII stages in canine oocytes.