The morphological characteristics of endometrium on day 6 after ovulation of conception (group 1) and non-fecund, menstrual (group 2) cycles have been studied in the rhesus monkey (n = 30). A conception cycle was distinguished by the presence of a developmentally normal, age-stage-synchronized embryo. Thus, 78% of the mated cycles (n = 18) yielding synchronous embryos (12 zona-encased and two zona-free blastocysts) were used for this study. On day 6 after ovulation, no significant changes in the serum concentrations of oestrogen and progesterone were seen in conception cycles (n = 14) compared with the non-mated, normal ovulatory cycles (n = 12). Morphometric analyses revealed that on day 6 of gestation (n = 8), endometrium differed from the corresponding non-mated luteal phase (n = 7) with significant increases in epithelial mitosis (P < 0.01), height of glandular epithelium (P < 0.05), volume ratio of gland cell to gland (P < 0.03), degree of pseudostratification (P < 0.02), and higher frequency of supranuclear, adluminal accumulation of vacuoles in gland cells (P < 0.05). The degree of stromal oedema was higher (P < 0.02) in fecund cycles but there was no change in venular diameter. In a separate set of experiments, estimates of tissue vascular response revealed a higher (P < 0.02) endometrial extravascular albumin space on the same day of gestation; there were no differences, however, in endometrial blood volume, or in the number of von Willebrand antigen-positive capillaries and small vessels between the two groups (group 1, n = 6; group 2, n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)