When compared with remifentanil, alfentanil postoperatively produced a more marked depression in respiratory rate and a slightly slower recovery from the anaesthetic, but at the same time, allowed for less analgesic supplementation for these patients. Transient hypertension following the termination of remifentanil infusion for neurosurgical procedures is a known phenomenon , but to our knowledge, there are no controlled randomised trials available to document this. The suggested mechanism of acute opioid tolerance is controversial, and other theories, such as NMDA receptor mediation, hyper-analgesia, or rebound effect remain to be verified. More recently, interactions between propofol and opioids have been targeted as areas of further research. We concluded that there is a need to demonstrate, whether there is a clinically significant difference in the occurrence of hypertension after remifentanil or alfentanil infusions, which may have implications on their use in a patient group where acute postoperative blood pressure elevations can have severe consequences. Both drugs that we aim to compare are safe, widely accepted and have been used for neurosurgical operations for more than 10 years. They are in use in our hospitals every day and our anaesthetists are familiar with them. We would like to collect accurate and detailed data to help us assess, whether there is any significant difference in recovery from anaesthetics using one or the other analgesic for neurosurgical procedures. We will only be looking at the period immediately after the operation whilst the patient is in the recovery unit, which is usually not longer than 3 hours. The data collected include regular measurements of blood pressure, heart rate, rate of breathing, level of oxygen saturation of the blood, body temperature, pain and nausea scores and level of alertness. Collection of the necessary data can be achieved without much difficulty or inconvenience to both patients and staff, as all of the information we need is already collected normally and shall not delay your recovery in any way. All patients will receive their regular and “as required” medications as usual. For scientific purposes we need to randomize (allocate to two comparable groups) every patient and standardize the treatment each group will receive. To enable the nursing staff to remain unbiased, they will be asked to carry out their regular observations without knowing which group each patient belongs to. We aim to recruit approximately 120 patients your participation will be greatly appreciated. We do not plan for any additional follow-up procedures as a result of this research.