The main limitation in producing in vitro dog embryos succesfully is the low oocyte maturation rate to the Metaphase II stage. The objective of this experiment was to compare the rates of nuclear maturation of dog oocytes cultured in Tissue Culture Medium 199 (TCM 199) supplemented with polyvinyl-pirrolidone (PVP) with oocytes cultured in TCM 199 with estrous cow serum (ECS) or TCM 199 with ECS and hormones. Ovaries were collected from 21 healthy bitches by ovariohysterectomy. Females were at various stages of the estrous cycle at the moment of ovary retrieval. The oocytes were selected and classified subjectively according to their morphology and size and matured for 48 hours at 37oC in an atmosphere of 5% CO2 in air. Oocytes were randomly allocated to one of three treatment groups: (A) TCM 199 supplemented with 4 mg/ml PVP, (B) TCM 199 with 10% ECS, or (C) TCM 199 supplemented with 10% ECS + 20 µg/ml estradiol + 0.5 µg/ml FSH + 0.03 IU/ml hCG (control). All maturation media contained a final concentration of 1 µg/ml of human somatotropin (hST). There were no significant differences in Metaphase II rates among treatments A (4.7%, 8/170), B (3.52%, 6/183), and C (4.70%, 8/172). It was concluded that in vitro nuclear maturation of domestic dog oocytes can be achieved using TCM 199 supplemented with PVP, and this yields results similar to those obtained using media containing serum, gonadotropins, or estradiol.