http://dx.doi.org/10.4025/actascihealthsci.v36i1.14472 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 24th, 48th and 72nd hour after the injury. PTU has reduced fibrinogen levels by 20% at the 24thh and by 30% at the 48thh (p < 0.001) and haemoglobin reduction at the 72nd hour (p < 0.001), which had already occurred during the 2nd collection in the other groups. CTU favoured erythrocyte reduction at the 48th 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.