Thyroid hormones--particularly triiodothyronine, T3--play a critical role in the morphological transformations comprising metamorphosis in larval bullfrogs (Rana catesbeiana). Traditional staging criteria for anuran larvae incompletely distinguish physiological and behavioral changes during growth. We therefore first developed a new parameter to describe larval growth, the developmental index (DI), which is simply the ratio between the tail length of the larva and its head diameter. Using the DI we were able to identify two distinct populations classifying the larvae during growth along a continuous linear scale with a cutoff value of DI at 2.8. Classification based on the DI, used in this study, proved an effective complement to existing classifications based on developmental staging into pre- or pro-metamorphic stages. Exposure to T3 in the water induced a rapid (beginning within 5 min) and significant decrease (approximately 20-40%) in locomotor activity, measured as total distance traversed and velocity. The largest decrease occurred in more developed larvae (DI<2.8). To determine correlated changes in the neuromuscular junctions during metamorphosis and apoptotic tail loss, miniature endplate currents from tail muscle were recorded during acute exposure to a hypertonic solution, which simulates an apoptotic volume decrease. Our results support a role for T3 in regulating larval locomotor activity during development, and suggest an enhanced response to volume depletion at the neuromuscular junction of older larvae (DI<2.8) compared to younger animals (DI> or =2.8). We discuss the significance of the possible role of an apoptotic volume decrease at the level of the neuromuscular junction.