We investigated the effects of variation in ambient air temperature on seed production in the field during the reproductive development phase of `Salinas' head-type lettuce (Luctuca sativa L.) in the central San Joaquin Valley of California where daytime maxima may exceed 38C for many consecutive days during reproduction. Florets were tagged daily for 41 days and harvested seeds were sampled to determine temperature-sensitive periods during seed development. The number of seeds per inflorescence (NOS), seed mass (SM), and seedling root length (SRL) were reduced and percentage germination (GERM) increased with increasing minimum (LT) and maximum (HT) temperatures. Daily HT > 35C greatly reduced NOS. Increasing LT reduced SM and SRL, but to a lesser extent than NOS (r2 = 0.23 and 0.40; P = 0.01 and 0.001, respectively). The advantage of increasing HT on GERM (r2 = 0.20; P = 0.01) was overshadowed by the severe reduction in NOS and the vigor components SM and SRL. The periods of greatest sensitivity to high air temperature for NOS, SM, GERM, and SRL were - 1 to +1, - 4, +1, and - 4 to - 3 days from anthesis, respectively. The hours of peak sensitivity for these variables occurred during the same days at - 36, - 101, + 15, and - 83 hours from anthesis, respectively. Using Box-Jenkins time series analysis, diurnal periodicity in temperature sensitivity for the four variables was determined.