The study investigated the expression of genes for progesterone receptor (PR) and for the cytokine leukaemia inhibitory factor (LIF) in the uterine tube and uterine horn tissues from pregnant and non-pregnant bitches. The aim was to study whether a relation existed between the likely biological effectiveness of progesterone (P-4) and the change in the uterine expression of LIF mRNA during pregnancy, as has been described in primates. For this purpose, 20 pregnant bitches were ovario-hysterectomized after being allotted to three groups according to gestational age (pre-implantation: days 10 to 12, n = 7; peri-implantation: days 18 to 25, n = 7; post-placentation: days 28 to 45, n = 7). Tissue samples were obtained from the uterine tubes, one uterine horn (including placentation sites and interplacental sites in bitches that had already implanted) and the corpus uteri, stored at -80 degrees C, and then analysed by qualitative and quantitative PCR for PR and LIF mRNA expression. From the pre-implantation to the placentation stage, a decrease in the relative expression of PR mRNA in uterine tissue was obvious and significant when expressed relative to beta-actin (11.2 +/- 6.8 vs 2.7 +/- 1.9; p < 0.05). However, over the same period, the relative expression of LIF mRNA increased (10.1 +/- 16.1 vs 50.0 +/- 32.3; p < 0.05). In addition, PR mRNA went from being detectable to no longer detectable in the uterine tube, and no longer detectable in interplacental-site uterine tissue. We conclude that LIF is important for the establishment of canine pregnancy; that decreased uterine PR mRNA expression may contribute to the increase in uterine LIF mRNA; and, that the ability of the embryo to preserve PR mRNA expression at implantation and placentation sites while expression is lost in the remainder of the uterus represent an effect important to the establishment and maintenance of pregnancy. We additionally propose that canine embryo secretory proteins have a regulatory effect on both PR and LIF before as well as at and after implantation.