Using a high-speed digital optical system, we determined the propagation characteristics of two leader/return-stroke sequences in the bottom 400 m of the channel of two lightning flashes triggered at Camp Blanding, Florida. One sequence involved a dart leader and the other a dart-stepped leader. The time resolution of the measuring system was 100 ns, and the spatial resolution was about 30 m. The leaders exhibit an increasing speed in propagating downward over the bottom some hundreds of meters, while the return strokes show a decreasing speed when propagating upward over the same distance. Twelve dart-stepped leader luminosity pulses observed in the bottom 200 m of the channel have been analyzed in detail. The luminosity pulses associated with steps have a 10--90% risetime ranging from 0.3 to 0.8 μs with a mean value of 0.5 μs and a half-peak width ranging from 0.9 to 1.9 μs with a mean of 1.3 μs. The interpulse interval ranges from 1.7 to 7.2 μs with a mean value of 4.6 μs. The step luminosity pulses apparently originate in the process of step formation, which is unresolved with our limited spatial resolution of 30 m, and propagate upward over distances from several tens of meters to more than 200 m, beyond which they are undetectable. This finding represents the first experimental evidence that the luminosity pulses associated with the steps of a downward moving leader propagate upward. The upward propagation speeds of the step luminosity pulses range from 1.9¿107 to 1.0¿108 m/s with a mean value of 6.7¿107 m/s. In particular, the last seven pronounced light pulses immediately prior to the return stroke pulse exhibit more or less similar upward speeds, near 8¿107 m/s, very close to the return-stroke speed over the same portion of the channel. On the basis of this result, we infer that the propagation speed of a pulse traveling along the leader-conditioned channel is primarily determined by the channel characteristics rather than the pulse magnitude. An inspection of four selected step luminosity pulses shows that the pulse peak decreases significantly as the pulse propagates in the upward direction, to about 10% of the original value within the first 50 m. The return-stroke speeds within the bottom 60 m or so of the channel are 1.3¿108 and 1.5¿108 m/s for the two events analyzed, with a potential error of less than 20%. ¿ 1999 American Geophysical Union