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As part of a health and safety assessment of ultrawideband sources, it was useful to determine stimulation thresholds for an electrically excitable tissue down into the low nanosecond range. Stimulation thresholds were measured using gastrocnemius muscles isolated from 16 frogs (Rana sp.). Single pulses were delivered with a pair of surface electrodes, and muscle twitch was measured with an isotonic transducer. Pulse durations of 100, 10, and 1 ms; 100, 10 and 1 μs; and 100 and ≈1 ns were used. Tissue voltage and current strength-duration (S-D) curves on log-log plots had a classic appearance, with thresholds for ultrashort pulses being linear. For a pulse of ≈1 ns, the mean threshold voltage in the muscle was 4.5 kV and the mean threshold peak current was 35 A. When delivered by direct contact, a single ultrawideband pulse of ≈1 ns could reliably produce a biological effect, stimulation of an electrically excitable tissue. The observation that the S-D curves extended downward to ≈1 ns in a linear manner suggested that classical ion channel mechanisms regulated excitation and that other processes, such as electroporation, did not occur. Although a single nanosecond pulse delivered by direct contact can elicit a biological response, such a stimulus in air is unlikely to produce an effect.