Although Ringer's lactate (RL) is routinely used for resuscitation, it is not known whether this fluid alone restores and maintains the depressed cardiac output (CO) following severe hemorrhage. To study this, a fiberoptic catheter was inserted to the level of the aortic arch in rats. Following indocyanine green (0.05 mg) administration, CO was measured using an in vivo hemoreflectometer (IVH). The rats were then bled to and maintained at a mean arterial pressure (MAP) of 40 mmHg until 40% of the shed blood volume was returned in the form of RL. They were resuscitated with 2, 3, or 4 times (x) the volume of the shed blood with RL and CO recorded at various intervals thereafter. The results indicate that CO decreased significantly during hemorrhage and remained depressed following resuscitation with 2 or 3x RL. CO was normal immediately after resuscitation with 4x RL, but it was not sustained and decreased significantly 0.5 to 8 hr postresuscitation. This was not due to the decreased hematocrit since acute hemodilution did not decrease CO. These results indicate that: (1) the progressive changes in CO following hemorrhage and resuscitation can be measured in rats by using IVH; (2) resuscitation with 4x RL restores total peripheral resistance to normal, but does not maintain CO, suggesting that pharmacological support may be needed under such conditions; (3) the lack of maintenance of CO following resuscitation may play an important role in the development of multiple organ failure after severe hemorrhage. © 1991.