Induction of ovulation post partum is associated with a high incidence of prematurely regressing corpora lutea. However, inadequate luteal function is not the sole reason for pregnancy failure, because ewes with normal corpus luteum function and successful fertilization also fail to establish pregnancies. The effects of suckling status and the interval from post partum to rebreeding on corpus luteum and endometrial function were examined in vivo and in vitro. Ewes were weaned early or allowed to lactate, induced to ovulate using a progesterone-impregnated controlled internal drug release device and an intramuscular injection of pregnant mare serum gonadotrophin, and inseminated (intrauterine) at either 21 or 35 days post partum (n = 10 per group). A further 10 standard ewes whose interval from parturition was in excess of 150 days were included for comparative purposes. On Day 10 after insemination the pregnancy rate was determined in four ewes from each of the post-partum groups and five standard ewes. These ewes were then ovariectomized and hysterectomized for studies in vitro. The incidence of premature luteal regression, as assessed by progesterone concentrations in peripheral blood was independent of the suckling stimulus but dependent on stage post partum (21 days post partum, 6 of 19 ewes; 35 days post partum, 0 of 19 ewes; P less than 0.05). Luteal function was normal in all standard ewes. Ovulation rate, corpus luteum weight, corpus luteum progesterone content and basal progesterone production in vitro were significantly less in 21-day than in 35-day post-partum ewes. Pregnancy rates as determined on Day 10 or at term were low in all post-partum groups (7 out of the 38 ewes inseminated) compared with standard ewes (8 of 10). Uterine function was assessed by culturing endometrial tissue from the tip and body of each uterine horn in the presence of [3H]leucine for 30 h at 37 degrees C. Incorporation of radiolabel into non-dialysable proteins synthesized and secreted by the endometrium in vitro was independent of uterine horn location and suckling status but was significantly lower (P less than 0.001) in media from 21-day than from 35-day post-partum ewes. Irrespective of treatment group, incorporation of radiolabel was positively correlated with mean plasma progesterone concentrations on Days 2-10 after insemination and with basal progesterone production in vitro. Secreted proteins were detected by two-dimensional-polyacrylamide-gel electrophoresis and fluorography.(ABSTRACT TRUNCATED AT 400 WORDS)