The objective of this study was to clarify the role of oestradiol in luteal function by examining its effect on the oxytocin stimulation of 15-keto-13,14-dihydro-prostaglandin F2 alpha (PGFM) concentrations in cyclic mares. In the first experiment, three groups of mares (4 per group) were given a bolus injection of 17 alpha-oestradiol (1 mg), oestradiol (1 mg) or vehicle on days 7, 9, 11, 13 and 15 of the cycle. Six hours later the mares were challenged with 10 iu oxytocin intravenously and frequent blood samples were taken from 15 min before to 15 min after for measurement of PGFM. Results showed a significant stimulatory effect of oestradiol (five times greater than controls at day 11; P < 0.05), but not of 17 alpha-oestradiol, on the oxytocin stimulation of PGFM. As a relatively large dose was given systemically in this experiment, a second experiment was performed to introduce a dose that was more physiological into the uterus. Small Silastic spheres (1 cm diameter) were impregnated with or without oestradiol at a concentration that gave a release rate similar to that of embryos at day 12 (10 ng h-1). These were inserted (one per mare) into the uterus of two groups of mares (five per group) on day 7. The mares were challenged with oxytocin on days 9, 11, 13 and 15 of the cycle and blood samples were taken as before for determination of PGFM. The results showed that oestradiol enhanced (four times greater than controls at day 13; P < 0.05) the oxytocin stimulation of PGFM concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)