Modified radical mastectomy (MRM) may be associated with severe post-operative pain, leading to chronic pain syndrome. We compared the post-operative analgesic profile of two ultrasound-guided nerve blocks: Paravertebral block (PVB) and serratus plane block (SPB). This double-blind, randomised study was conducted on fifty adult females, scheduled for MRM with axillary dissection. After inducing general anaesthesia with intravenous midazolam 1 mg, fentanyl 1.5 mcg/kg, propofol 1-2 mg/kg and vecuronium 0.1 mg/kg, patients were administered either ultrasound-guided thoracic PVB at T4 ( = 25) or SPB at 5 rib ( = 25) with 20 ml of 0.5% bupivacaine, both as a single level injection. Time to first rescue analgesia and morphine consumption in 4, 6, 24, 48 and 72 h by PCA pump, visual analogue scale score and any adverse effects were recorded. Quantitative variables were compared using the unpaired -test or the Mann-Whitney U test between the two groups. Qualitative variables were compared using the Chi-square test or Fisher's exact test. The duration of analgesia (mean ± Standard deviation SD) was significantly longer in the PVB group compared to SPB group (346 ± 57 min vs. 245.6 ± 58 min, < 0.001). The post-operative 24 h morphine consumption (mean ± SD) was significantly higher in the SPB group (9.7 ± 2.1 mg) compared to PVB group (6.5 ± 1.5 mg) ( < 0.001). Ultrasound-guided SPB is an alternative analgesic technique to thoracic PVB for MRM although PVB provides a longer duration of analgesia.