The effect of preovulatory endocrine changes upon oocyte maturation was investigated in a combined in vivo and in vitro study. A preliminary study was performed to investigate the effect of endocrine influences in vivo on subsequent oocyte maturation in vitro. A hemi-ovariectomy was performed in four bitches during pro-oestrus at the first detected rise of plasma progesterone (mean progesterone 2.36 +/- 0.58 ng ml-1) and a second performed during oestrus immediately before ovulation, 1-4 days later (mean progesterone 4.96 +/- 0.75 ng ml-1). The mean numbers of high quality (grade 1) oocytes with identifiable nuclear material collected per pro-oestrous ovary was 15.5 +/- 9.88, and per oestrous ovary was 31.75 +/- 23.76. All of the grade 1 oocytes harvested were cultured for 96 h, after which 37% and 19% of pro-oestrous oocytes had matured to germinal vesicle breakdown (GVBD) and metaphase I/anaphase I/metaphase II (MI/AI/MII), respectively, whereas 41% and 11% of oestrous oocytes had matured to GVBD and MI/AI/MII, respectively. There was no significant difference in maturation between the two groups of oocytes. In a second study, high quality grade 1 oocytes harvested from bitches in the late luteal or anoestrous stage of the oestrous cycle were cultured in medium supplemented with either 1 microgram oestradiol ml-1 (64 oocytes), 1 microgram progesterone ml-1 (52 oocytes), a combination of oestradiol and progesterone (50 oocytes), or no supplementation (55 oocytes). The mean numbers of high quality (grade 1) oocytes with identifiable nuclear material collected per luteal ovary was 9.71 +/- 7.96, and per anoestrous ovary was 8.33 +/- 5.61. The number of oocytes obtained per bitch was highly variable, and may have been affected by the stage of oestrous cycle. After 48 h, maturation to GVBD was 56, 33, 50 and 50%, and to MI/AI/MII was 13, 17, 20 and 4% for each treatment respectively, and after 96 h was 45, 59, 63 and 63% and 10, 0, 4 and 0%, respectively. There was no significant difference in maturation between the four groups. The hormonal environment in vivo did not affect subsequent in vitro maturation of high quality oocytes. Similarly, culture in steroid hormone supplemented medium did not improve maturation in vitro of high quality oocytes. The effects upon oocytes from specific preovulatory follicles were not studied.