1. The role of satellite cells in muscle growth during development is well documented, but the involvement of these cells in muscle repair after contusion is less well known. In the present study, we investigated the time-course of satellite cell activity (from 3h to 7days) after contusion of rat gastrocnemius muscle using specific molecular markers for immunofluorescence and real-time polymerase chain reaction (PCR). 2. Inflammation of the injured muscle occurred within 6h, followed by disintegration of the damaged myofibres within 12h. Newly formed myofibres appeared by Day 7. 3. The number of MyoD-positive nuclei (activated satellite cells) in the injured muscle was significantly increased by 6h, reaching a maximum by 12h after contusion. However, the number of MyoD-positive nuclei decreased towards control levels by Day 7. Changes in the number of bromodeoxyuridine-labelled nuclei (proliferating satellite cells) paralleled the changes seen in the number of MyoD-positive nuclei. Conversely, expression of myogenin protein was not apparent until Day 3 and increased further by Day 7. Colabelling of MyoD and myogenin was seen in only a few cells. 4. The time-course of MyoD mRNA expression corresponded with MyoD protein expression. However, there were two peaks in myogenin mRNA expression: 6h and Day 7 after contusion. The second peak coincided with upregulation of myostatin mRNA levels. 5. The results of the present study suggest that contusion activates a homogeneous population of satellite cells to proliferate within 3days, followed by differentiation to form new myofibres. The latter may be regulated, in part, by myostatin.