This study investigated the effects of ultrasound therapy on haematological dynamics and plasma fibrinogen during the inflammatory phase of muscle injury. Forty-eight male Wistar rats were divided into control group (CG), continuous ultrasound treated group (CTU) and pulsed ultrasound (PTU). Animals were subjected to surgical incision. A transverse lesion was made in the biceps femoris muscle (50%). CTU (1.0 MHz) was applied at 12-hour intervals on the lesion, for three days, with 0.4 W cm -2 and three minutes of duration (six applications in the total). PTU was applied in the pulsed mode 20% (2 ms on/8 ms off), maintaining the other parameters. Fibrinogen, white and red blood cells were analyzed in the 24 th , 48 th and 72 nd hour after the injury. PTU has reduced fibrinogen levels by 20% at the 24 th h and by 30% at the 48 th h (p < 0.001) and haemoglobin reduction at the 72 nd hour (p < 0.001), which had already occurred during the 2 nd collection in the other groups. CTU favoured erythrocyte reduction at the 48 th h (p = 0.003). PTU presented an anti-inflammatory effect due to plasma fibrinogen reduction, and CTU favored haemorrhage due to the reduction of erythrocytes when applied in the first 72 hours after muscle injury. Efeitos do ultrassom terapêutico na dinâmica hematológica e no fibrinogênio durante a fase inflamatória após lesão muscular em ratos RESUMO. O objetivo desta pesquisa foi investigar os efeitos do ultrassom terapêutico na dinâmica hematológica e no fibrinogênio plasmático durante a fase inflamatória de lesão muscular. Quarenta e oito ratos Wistar foram divididos em grupo controle (CG), grupo tratado com ultrassom contínuo (CTU) e tratado com ultrassom pulsado (PTU). Os animais foram submetidos a uma incisão cirúrgica. A lesão transversal (50%) foi realizada no músculo bíceps femoral. O CTU (1,0 MHz) foi aplicado sobre a lesão em intervalos de 12h, por três dias, a 0,4 W cm -2