An investigation into the genetic basis of possible differences in the activity of isloated chloroplasts was conducted with 5 maize inbreds (WF9, R151, R177, R181, and Oh43) and 7 hybrids (WF9 ✕ Oh43, WF9 ✕ Rl51, WF9 ✕ R181, R151 ✕ R177 and reciprocals of the last 3). The ability of the chloroplasts of the various inbreds and hybrids to carry out the Hill, cyclic and noncyclic photophosphorylation recations was demonstrated. These reactions were found to vary in the same manner from genotype to genotype; thus one reaction could serve as an indicator for all three. After establishment of this relationship, the cyclic photophosphorylation reaction was used as the criterion for chloroplast activity.Of the inbreds tested, chloroplasts isolated from R151 had the highest, and those from WF9, the lowest activity. In most instances the chloroplasts from the F1 hybrids had activities intermediate between those of their respective inbred parents. No case of heterosis with respect to activity was observed, although one F1 cross (R151 ✕ R177) was significantly lower in activity than the mean of its parental inbreds. There were no significant differences between the reciprocal crosses.