Systemic treatment with epidermal growth factor (EGF) induces growth of all wall layers in the urinary tract of pigs and rats. The present study was initiated to describe morphological and biochemical changes in the bladder smooth muscle from rats treated with EGF for 4 weeks. Eight-week-old female Wistar rats were treated with subcutaneous injections of vehicle (n = 16) or EGF (n = 8, 150 micrograms/kg per day) for 4 weeks. After EGF treatment the bladders were increased in weight [74.4 +/- 0.4 vs 122.1 +/- 0.5 mg, P < 0.001 (mean +/- SEM)]. Sodium dedecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) analyses of six bladders from each group revealed that the total amounts of actin, myosin and desmin were statistically significantly increased by 62%, 61% and 154%, respectively. The relative amounts of actin and myosin were unchanged whereas the desmin to actin ratio was significantly increased - as previously described in rat bladder smooth muscle hypertrophy. Light and electron microscopy of two bladders from each group revealed increased wall thickness involving all wall layers. The smooth muscle fibres at a midventral bladder location seemed only slightly hypertrophic-some degree of hyperplasia was therefore suspected. In conclusion, EGF treatment for 4 weeks induced a net synthesis of contractile and cytoskeletal proteins in the urinary bladder smooth muscle.