A field trial of cowpeas (Vigna unguiculata (L.) Walp.) was performed in the dry season at Kano in Sudan Savanna of Nigeria. Grain yield of highly drought-tolerant lines (TVu 11979, 11986, 12348) was around 1 t/ha, while that of highly susceptible ones (TVu 7778, 8256, 9357) was only about 0.2-0.3 t/ha. The productivity of 1 t/ha was similar to that recorded in the rainy season at Kano. This trial confirmed the results of previous evaluation of drought tolerance of cowpea lines. It also suggested that highly tolerant lines could be cultivated in the dry season in farmers' fields. Two cowpea lines (TVu 11979: highly tolerant to drought, TVu 9357: highly susceptible to drought) were cultivated in pots at Tsukuba, Japan. Half of the pots were subjected to water stress (hereafter referred to as stressed plot) and the others were watered periodically (control plot). In the stressed plot, dry matter increase per plant was markedly and equally reduced in both lines and no difference was observed between the highly tolerant line and the highly susceptible one. The distribution of dry matter among plant organs, however, was different at the ripening stage between the two lines. In the highly tolerant line, a larger proportion was distributed to roots, and a smaller proportion to pods in comparison with the highly susceptible one. This characteristic of highly tolerant lines seemed to be very advantageous for collecting residual water from deep soil layers and for dry matter production in the dry season. However, when the lines were planted in pots, the root zone was limited. The vigorous growth of roots of these tolerant lines, therefore, could be ineffective in collecting water in potted plants.