 This is a prospective, single centre, two-part, three doses study. Part 1 is a Phase I, three cohorts, dose-ascending, open-label, safety study. Part 2 is a Phase II, randomised, parallel-group, double-blind, placebo-controlled, exploratory efficacy and safety study The objective of the study is to investigate the efficacy and safety of a single intrathecal injection of paracetamol, administered at 3 doses to 3 active treatment groups, as compared to placebo solution, for post-operative analgesia of hip replacement surgery performed under spinal anaesthesia. Patients scheduled for hip replacement surgery will be randomised into 4 treatment groups 15 patients per group to receive either one of the 3 single doses of paracetamol 3 D1: 60 mg, D2: 90 mg, D3: 120 mg or placebo solution P: saline solution by intrathecal IT injection. Immediately after paracetamol or placebo IT administration, all patients will receive a single IT dose of Hyperbaric Bupivacaine HCl 0.5 12.5 mg for 160 cm-tall patients and 15 mg for 160 cm-tall patients. The time interval between paracetamol IT and bupivacaine IT administrations should not exceed 2 min. 