Maize [Zea mays L.] hybrids with tropical genetic background are a promising source of heat stress tolerance, but their performance in high yielding environments remains poorly understood. Our objective was to assess [i] genotypic differences in the ecophysiological determinants of grain yield; i.e., fraction of light intercepted by crop [fIPAR], radiation use efficiency for biomass production [RUE], and harvest index [HI], and [ii] the responses of mentioned traits to brief episodes of high temperature. The contribution of stored reserves to grain yield was also analyzed. Field experiments included three contrasting maize hybrids [Te: temperate; Tr: tropical; TeTr: Te×Tr] grown under two temperature regimes [control and heated] during daytime hours. We tested heating [ca. 33-40°C at ear level] along three 15-d periods [GS 1: pre-silking; GS 2: from silking onwards: GS 3: active grain filling]. Heat stress had no effect on leaf area and fIPAR, but heating during grain filling affected light capture through reduced cycle duration, especially for the Te hybrid [average of -16.5 d]. Heating caused a large reduction in RUE, but this trait had a rapid recovery after heat removal and final shoot biomass was not much affected [between -3 percent and -33 percent]. HI was markedly reduced by heating and its variation was associated with changes in reserves use [r 2=0.61]. Grain yield in heated plots was better explained [r 2 greater than or equal to 0.92] by the variation in HI than by the variation in final shoot biomass [r 2 greater than or equal to 0.59]. Heat effects on grain yield were larger [i] when they occurred around flowering [-527gm -2 for GS 1 and -545gm -2 for GS 2] than during grain filling [-352gm -2 for GS 3], and [ii] for the Te hybrid [-599gm -2] than for the TeTr [-440gm -2] and the Tr hybrids [-384gm -2]. Heating around silking [GS 1 and GS 2] caused apparent accumulation of reserves during the effective grain-filling period. The opposite trend was detected among plots heated during active grain formation [GS 3]. The tropical genetic background did not penalize yield potential and conferred an enhanced capacity for enduring heat effects.