The rate of cerebrospinal fluid (CSF) formation (Vf) and resistance to reabsorption (Ra) of CSF were determined in dogs at four doses of fentanyl (0.05, 0.18, 0.60, and 3.0 microg.kg. min), sufentanil (0.01, 0.04, 0.13, and 0.60 microg.kg min) and aflentanil (1.4, 4.0, 13.0, and 40.0 microg.kg min). Results were compared within and between groups and to previously reported normal values (obtained during a variety of background anesthetics) for Vf (0.030-0.054 ml/min) and Ra (220-253 cm H2O ml min) in dogs. At the two lower doses of fentanyl and at all doses of sufentanil and alfentanil, Vf values were not significantly different from previously reported normal values. At the two higher doses of fentanyl, Vf decreased by 24 and 49%, respectively. At the two lower doses of all three drugs, Ra was significantly decreased, with mean values 40-52% below previously reported normal values. At the two higher doses of alfentanil, Ra values were not significantly different from previously reported normal values, and at the two higher doses of fentanyl and sufentanil, Ra was unchanged or increased. It is concluded that, among these three narcotics, reduction of CSF volume (as determined by the balance between Vf and Ra) is favored most by fentanyl, and ease of CSF volume contraction (as determined by Ra) is favored most by alfentanil.