We have measured the cross section [σAl=4.2×10-17cm2±36％] for associative ioniza-tion occurring in collisions between two Na(3P) atoms, by measuring the current reselting from excitation of Na vapor with CW-laser radiation. The major experimental difficulty in measurements of this type is in the determination of the number density and spatial distribution of excited atoms. Here we have measured the excited-atom density by three independent methods, and we have studied the spatial distribution of excited atoms in detail. We have compared the cross section obtained by us at =523k with other experimental results.