Study aim: determinaton of the analgesic effect of transversus abdominis plane block following appendectomy Design: A single-center, two-arm, parallel-group, randomized, active-controlled, double-blind clinical trial will be conducted at Imam Reza Hospital in Birjand, Iran. The target sample size is 60 patients, with a target of 30 patients per group. Patients will be assigned to either a TAP block with ropivacaine or intravenous pethidine using simple computer-generated randomization with equal allocation probabilities (1:1). Participants and data analysts will be blinded to group allocation. Settings and conduct: Single-center, two-arm, parallel-group, randomized, active-controlled, double-blind clinical trial will be conducted at Imam Reza Hospital in Birjand, Iran Participants/Inclusion and exclusion criteria: include: age over 15,asa score 1 and 2 exclude:perforation,appendicitis with hernia,bowel surgery,gyno surgery with appendectomy, age under 15 Intervention groups: The intervention group will receive a single ultrasound-guided, right-sided lateral transversus abdominis plane (TAP) block administered by an anesthesiologist at the end of open appendectomy, before emergence from anesthesia. Twenty milliliters of 0.25% ropivacaine will be injected between the internal oblique and transversus abdominis muscles. The control group will receive 25 mg of intravenous pethidine before transfer to the recovery room. Rescue analgesia will be provided as needed in both groups according to the study protocol. Main outcome variables: Pain intensity will be measured using the Numerical Rating Scale at 1, 2, 6, 12, and 24 hours after surgery. Nausea and vomiting, adverse events, time to the first request for analgesia, and analgesic consumption will be recorded during the first 24 hours. Rescue analgesia will be provided as needed in both groups according to the protocol