A number of collections of maize from Guatemala and other Central and South American countries have shown varying photoperiodic responses when grown on the longer summer days at Ames, Iowa. A few lines mature normally, most flower weakly in September or October, too late to mature, and a few have shown no flowering when grown outdoors in Iowa;Covering young corn plants in the field with light-tight boxes to shorten the days to 11 or 12 hours during June or July has hastened and increased flowering of all of the lines tested, even of early Iowa corns. The gain in one Guatemalan corn was 31 days in time of first pollen shed, and percentage of stalks shedding pollen was increased from 2 to 96 percent. Covering for four weeks starting four weeks after emergence was most successful of the treatments tried. When given a 4-week photoinduction period, two Guatemala corns were three or four weeks earlier in Iowa than they normally are at high altitudes and lower temperatures in Guatemala;Chemical analyses of carbohydrate and nitrogen fractions of corn plants receiving long- and short-day exposures showed no significant differences;Injections of 2,4-dichlorophenoxyacetic acid into young corn plants growing under long days did not hasten the formation of floral primordia.