This report tests the hypothesis that intravascular prehydration with 3% gelatin in electrolyte solution maintains arterial blood pressure after spinal anesthesia better than with an equal volume of isotonic saline solution. Thirty-four patients undergoing elective transurethral resection of the prostate were allocated randomly to receive either 7 mL/kg of isotonic saline 0.9% (17 patients) or 7 mL/kg of 3% gelatin in electrolyte solution (17 patients) before spinal anesthesia. There was a significant increase in central venous pressure in the gelatin group without any significant change in the isotonic saline group. After spinal anesthesia, the mean systolic blood pressure significantly decreased in both groups; however, the incidence of systolic blood pressure greater than 75% of control value was higher in the gelatin group (15/17) than in the normal saline group (9/17). Also, the mean dose of phenylephrine required to maintain arterial blood pressure >75% of the baseline value was significantly larger in the normal saline group than in the gelatin group. We conclude that prophylactic administration of gelatin is more effective than saline in attenuating spinal anesthesia-induced hypotension.