Beginning at either 30 minutes, 2 hours, 4 hours, or 8 hours after 180 g compression of the cat L2 spinal cord for 5 minutes, infusion of U74006F was initiated. In this series, the cats received a total U74006F dose of 5 mg/kg/48 hours. An additional group of injured cats was treated at 8 hours postinjury with a three-fold higher dose of U74006F (i.e., a total 48-hour dose of 15 mg/kg). Controls received an equal volume of vehicle (citrate-buffered saline) delivered over 48 hours. The cats were evaluated weekly for 4 weeks for recovery of overground locomotion based on an 11-point scale by an investigator blinded to the time and type (i.e., vehicle or drug) of material administered. By 4 weeks postinjury, there was no significant difference in the locomotor recovery of cats that received U74006F at either 30 minutes, 2 hours, 4 hours, or 8 hours after injury. However, only recovery in the groups treated at 30 minutes, 2 hours, or 4 hours after injury was significantly greater than vehicle-treated controls. Locomotor function in cats receiving either 5 mg/kg/48 hours or 15 mg/kg/48 hours of U74006F at 8 hours postinjury was not significantly different from that of the vehicle-treated animals. Mean (+/- SEM) 4-week recovery scores were 6.8 +/- 0.9, 5.9 +/- 1.0, 7.2 +/- 1.1, and 4.7 +/- 2.9 out of 11 for cats treated at 30 minutes, 2 hours, 4 hours, or 8 hours postinjury, respectively, with the 5 mg/kg/48 hour dose. The mean recovery score for cats treated at 8 hours after injury with the 15 mg/kg/48 hour dose was 3.4 +/- 1.8. The average score for the vehicle-treated controls was 1.8 +/- 0.8. These findings demonstrate that U74006F can significantly protect locomotor function in our model of compression spinal cord injury if administered as late as 4 hours postinjury. Delaying administration of the compound to 8 hours after injury results in considerable loss of its protective capabilities even if the dose is increased threefold.