A rat model simulating some aspects of weightlessness was used to determine whether simulated weightlessness might alter interferon production. The optimum time for in-vivo induction of alpha/beta interferon (alpha/beta-IFN) by polyriboinosinic-polyribocytidylic acid was determined to be four hours in normal, mature rats. Rats suspended in the model for two weeks were injected with polyriboinosinic-polyribocytidylic acid and bled four hours later. A dramatic decrease (80%) in alpha/beta-IFN production was observed in those animals exposed to simulated weightlessness as compared to control rats. These data suggest that weightlessness may alter certain immunological functions.