The ASTROCULTURE™-1 (ASC-1) flight experiment, flown on STS-50 as part of the U.S. Microgravity Laboratory-1 mission, June 25 to July 9, 1992, successfully demonstrated the ability of the WCSAR porous tube nutrient delivery system (PTNDS) to control water movement through a particulate rooting matrix in microgravity. One critical aspect of this demonstration was to maintain the fluid circulating through the porous tubes at a slight negative pressure. Control of the fluid loop pressure allows regulation of the amount of water maintained in the rooting matrix while preventing free water from escaping the root zone in microgravity.Pressure control in the ASC-1 flight unit was achieved by using a digital microcomputer and a proportional-plus-integral-plus-derivative control algorithm to manipulate flow restrictors and pump speeds in response to changes in fluid pressure. The goal of the PTNDS control system used on ASC-1 was to maintain fluid pressure in the porous tubes within ± 0.5 cm H2O (± 49.1 Pa) of the pressure setpoint. Data collected during flight showed an actual control accuracy of approximately ± 0.8 cm H2O (± 78.5 Pa) over a series of pressure setpoints. During ground and flight tests, occasional control system oscillations were observed. Subsequent modification of the control algorithm eliminated these oscillations.Advanced configurations of the fluid loop pressure control system for future ASC flights will provide greater precision in pressure control and reduce mass, power and volume requirements.