The beneficial perioperative effects of the alpha-2 agonist clonidine have recently been verified in paediatric patients, and the pharmacokinetics have been found to be similar to what has been reported in adults. In a previous study we found that 2.5 micrograms/kg of clonidine causes a significant reduction in blood pressure. The aim of the present study was to evaluate if even lower doses of clonidine are associated with dose dependent or plasma-dependent haemodynamic changes. In a prospective, single, blind, controlled clinical trial, 24 paediatric patients (age range: 13-78 months) were randomised into three groups: control, intravenous clonidine 0.625 microgram/kg and intravenous clonidine 1.25 micrograms/kg respectively. Non-invasive blood pressure and heart rate were recorded at 3-min intervals for 30 min. Plasma concentrations of clonidine were analysed at 15 and 30 min post-injection. A decrease in MABP compared to baseline values was observed in all groups. A significantly greater reduction in blood pressure was seen in all groups receiving clonidine compared to control. In conclusion, low doses of clonidine (0.625 and 1.25 micrograms/kg) were found capable of causing a blood pressure reduction compared to control. We could not establish a plasma concentration dependent blood pressure effect. The observed blood pressure reductions caused by clonidine were of moderate magnitude. No effect on heart rate was observed, a finding which most likely is explained by the inclusion of atropine in the premedication.