Cyclic (n = 30) and pregnant (n = 29) Merino ewes were examined (n = 3 to 5 at most time points) over Days 0-16 and 0-22 after oestrus, respectively. As IGFBP activity was detected in some plasma and ULF samples, all samples were subjected to acid-gel chromatography before assay for IGF- I. After oestms, the overall means of both groups of ewes showed lower ULF IGF-I content (Days 3 and 12), lower plasma IGF-I concentrations (Days 3- 16), higher endometrial expression of mRNA encoding IGFBP-1 (Days 12-16) and lower endometrial expression of mRNA encoding IGFBP-2 (Day 8). Between Days 0 and 16 after oestrus, the pregnant ewes had lower plasma IGF-I concentrations and higher endometrial expression of IGFBP-1 mRNA than did the cyclic ewes. The presence of IGF-I in the ULF throughout the oestrous cycle and early pregnancy suggests a role of IGF-I in early pregnancy, influencing both uterine growth and embryonic survival. The concomitant endometrial expression of mRNA encoding IGFBP-1 and IGFBP-2 suggests a role of these binding proteins in the regulation of IGF-I bioavailability in the uterine environment of the ewe.