The mechanism by which one or more dominant ovarian follicles continue development while other subordinate follicles regress is not known. The mitogen activated protein kinases (MAPKs) are a group of kinases that are activated by hormonal factors and form a cascade of processes that regulate cell growth, division and differentiation. The aim of the present experiment was to characterise the presence of the MAPKs, Erk 1/Erk 2 and Akt in healthy dominant follicles and regressing subordinate follicles. Following in vivo monitoring of ovarian follicle development, three ewes were ovariectomised and the follicular fluid and follicle wall (theca and granulosa cells) saved from the dominant and largest subordinate follicle. The dissected diameter and follicular fluid oestradiol concentration of the dominant follicle was larger (P<0.01) than the largest subordinate follicle (6.5+/-0.0mm and 41.3+/-4.9ng/ml versus 4.7+/-0.3mm and 0.6+/-0.4ng/ml). Western blot analyses showed that there was more Akt (202.7+/-6.4 versus 59.6+/-32.7 units; P<0.05) and Erk 1/Erk 2 (104.5+/-10.6 versus 0.3+/-0.2 units; P<0.01) present in follicle wall samples from the dominant compared to the largest subordinate follicles. Phosphorylated forms of Akt and Erk 1/Erk 2 were detected in samples from dominant but not subordinate follicles. We suggest that signal transduction pathways involving Akt and Erk 1/Erk 2 may play an important role in determining the outcome of ovarian follicle growth and development in sheep.