The author carried out an experiment of watering of lettuce (Lactuca sativa L.) in the vegetable garden of the Escola Superior de Agricultura "Luiz de Queiroz", with soil of the type know as "terra roxa". Eight treatments, with 4 replications, were used, divided into 3 groups, as follows: Group A - Watering once a day, late, in the afternoon, with 5, 10 and 15 liters per square meter; Group B - Watering twice a day, with 10 and 15 liters per square meter, one half in the morning, one half late in the afternoon; Group C - Watering as in group A, but with a 0.1% Chilean nitrate of soda, every 3 days. The size of plots was 2,0 x 2,0 meters. The means obtained, with their respective standard errors, were the following: Group A- (4295 ± 53 gm.) 5 lit./sq.m. 4120 ± 92 gm. 10 lit./ sq. m. 4248 ± 92 gm. 15 lit./sq.m. 4518 + 92 gm. Group B- (4091 ± 65 gm.) 10 lit./sq.m. 3960 ± 92 gm. 15 lit./ sq. m. 4223 ± 92 gm. Group C- (4490 ± 53 gm.) 5 lit./ sq. m. 4300 ± 92 gm. 10 lit./sq.m. 4480 ± 92 gm. 15 lit./sq.m. 4690 ± 92 gm. Differences between groups, as well as within them were significant, even if within groups B the 5% probability level was not quite reached.