Domestic dogs are mono-oestrous and predominantly non-seasonal breeders with ovarian cyclicity unrelated to uterine function. During the first phase of dioestrus or pregnancy luteal function seems to be independent of gonadotrophic support and the gradual decline of progesterone concentration thereafter occurs in spite of an increased availability of prolactin and luteinising hormone (LH). Thus, the mean area under the curve (AUC) for LH (ng per 360 min ml−1) increased from 1672 in early dioestrus to 2510 in late dioestrus and from 2940 on day 20 to 5089 on day 55 of pregnancy. LH pulse frequency remained unchanged. An induced release of LH during each quarter of dioestrus had no immediate effect on progesterone concentration but production over the whole period of dioestrus was increased (P < 0.03). Results from blocking cyclooxygenase activity indicated that the rather precipitous luteolysis prior to parturition was PgF2α-mediated. During anoestrus LH pulse frequency did not change. The AUC for LH decreased from late dioestrus to early anoestrus (6332 vs. 3257 ng per 360 min ml−1) and increased from mid to the end of anoestrus (5830 ng per 360 min ml−1). Only during the terminal phase of anoestrus could resumption of ovarian activity be induced by treatment with oestradiol-17β. There was a depression of oestradiol-17β during mid anoestrus while it increased at the end of anoestrus.
