Mammalian gravity receptors (maculas) are morphologically organized for weighted, parallel distributed processing of information. There are two basic circuits: 1) highly channeled, type I cell to calyx; and 2) distributed modifying, type II cells to calyces and processes. The latter circuit should be the more adaptable since it modifies final output. To test this hypothesis, rats were flown in microgravity for 9 days aboard a space shuttle and euthanized shortly after landing. Hair cells and ribbon synapses from maculas of 3 flight and 3 ground control rats were studied ultrastructurally in blocks of 50 serial sections. Synapses increased by approximately 41% in type I cells and by approximately 55% in type II cells in flight animals. There was a shift toward the spherular form of ribbon synapse in both types of hair cells in flight animals (P < or = 0.0001), a near doubling of pairs in the flight rats (P < or = 0.0001), and an increase, by a factor of 12, in groups of synapses in type II cells (P < or = 0.0001). Current findings tend to support the stated hypothesis and indicate that mature utricular hair cells retain synaptic plasticity, permitting adaptation to an altered gravitational environment.