The present study investigates the effects of dosage and different modes of delivery of mouse epidermal growth factor (EGF) on the production of breaks in the fleece and on wool growth in Merino wethers. Subcutaneous infusions of EGF of greater than or equal to 0 . 25 mg kg-0.75 for 7-28 h resulted in a dose-dependent total or partial inhibition of wool production 2-4 weeks later. A complete break appeared in the fleece that was shed. Lower doses had lesser inhibitory effects on wool growth: the fleece was not shed but bore a zone of weakness, termed an incomplete break. Inclusion of the glucocorticoid analogue dexamethasone in the infusate did not alter the action of EGF on the fleece. Although a higher plane of nutrition increased the rate of fibre production, it did not alter the extent of inhibition of wool growth by EGF. Infusion of a peptide from EGF, which lacked eight of the C-terminal amino acids (EGF1-45), was as effective as the parent molecule in inhibiting wool growth. EGF administered as a single subcutaneous injection was less reliable as a method for producing breaks in the fleece. Of seven wethers that received EGF at a dose level between 0 . 27 and 0 . 32 mg kg-0.75, only three shed their fleeces. The remainder either developed incomplete breaks in the wool or were not affected. Administration of EGF at a dose level of 0 . 56 mg kg-0.75 via a rumen tube to one sheep had no discernible inhibitory effect on wool growth.