The levels of beta-endorphin and Leu-enkephalin were measured in the plasma of 23 children (16 male, 7 female) who were admitted to the Children's Hospital for elective (non-emergency) surgery, adenoidectomy and/or tonsillectomy. Two blood samples were obtained preoperatively. The first sample was taken during admission to the hospital; the second on 24 h later and just 5 min before the operation, without pre-medication or narcosis. beta-endorphin and Leu-enkephalin levels were measured using a RIA method after solid phase extraction of the samples. Mean beta-endorphin concentrations during admission to the hospital were 11.77 +/- 9.36 pg/ml. Twenty four hours later, and just 5 min before surgery, the mean levels of beta-endorphin were increased to 27.45 +/- 16.66 pg/ml. A statistically significant increase (t-value = -5.42) was found for beta-endorphin levels 5 min before the operation compared with 24 h earlier (2-tail probability p < 0.0001). The Wilcoxon matched-pairs signed-ranks-test also showed p < 0.0001 (Z = -4.197). In contrast, Leu-enkephalin levels remained unchanged. The mean values were 11.44 +/- 1.49 pg/ml on the day of admission, and 12.11 +/- 2.08 pg/ml 5 min before the operation. We did not find statistically significant differences between the paired samples (t-value = -1.53, 2-tail probability p = 0.139). The increase in beta-endorphin could be the result of preoperative anxiety about the narcosis and surgery.