Male frogs typically call near water at dusk to attract females for breeding. During the breeding season, male treefrogs also emit diurnal "tree calls" or "rain calls" from the tops of trees. Very little is known about tree calls, although many treefrogs use them. Tree calls may be used to attract females, deter males or be triggered by weather conditions favorable for breeding: high temperature and relative humidity, and a drop in barometric pressure. As dusk approaches, male treefrogs continue tree calls from lower in the trees, and if conditions are favorable, jump to the ground and travel to a nearby breeding pond where they begin their repetitive nocturnal mating calls. The scant published information is mostly descriptive and does not address the fitness benefit of calling from treetops far from breeding ponds. My goal was to determine the function of tree calls based on their environmental, behavioral, and acoustic characteristics. My data indicate tree calls are not rain calls. Each treefrog species that I studied (Hyla femoralis, H. gratiosa, and H. squirella) called most frequently at different combinations of mean environmental characteristics (temperature, relative humidity, and barometric pressure). Hyla femoralis and H. gratiosa gave tree calls at ambient air temperatures that differed significantly from the distributions recorded when no treefrogs called. Temperature, relative humidity, and barometric pressure distributions of calling activity differed significantly among all three species and from the distributions recorded when no treefrogs called. Hyla squirella called most often at a significantly different mean relative humidity of 1015 mbar; whereas H. gratiosa and H. femoralis called at a median1017 mbar. Means and fluctuations (summarized as SD) of the three weather parameters explained significant variation in tree calling activity (32-60%). Tree calling activity for all three treefrog species were also not significantly affected by subsequent rain. These results indicate that tree calls were not given at random with respect to environmental conditions. My data suggest tree calls are advertisement calls that deter males from an area, as evidenced by partitioning of tree calls among species during the day. In a playback experiment conducted at Chuluota Wilderness Area, Florida (28o38.31'N 81o07.24'W) no significant effect on mating behavior was found for either call indicating that neither tree calls alone or in conjunction with mating calls are necessary for mating . However, due to habitat differences between treatments and a limited number of experimental replicates, further research is needed. Preliminary results indicate an additional four natural ponds should be sampled to determine a possible effect for tree calls. Acoustic analysis showed that tree calls had fewer pulses per call, more time between pulses within a single call, and a higher minimum call frequency than mating calls. Call duration and maximum call frequency of tree and mating calls did not differ significantly. My research has greatly increased the information known about tree calls. My results indicate tree calls are not only "rain calls," a common misperception about daytime tree calls. However, more research is needed to fully understand the function of tree calls.