This study examined the effects of heat stress on composition of colostrum from primiparous cows during late pregnancy and the early postpartum period. Two groups of 6 Holstein heifers were utilized. During the last 3 wk of pregnancy and during the first 36 h after calving, one group was exposed to thermal comfort (temperature-humidity index = 65); the other group was exposed to high air temperatures (temperature-humidity index = 82 from 0900 to 2000 h and temperature-humidity index = 76 from 2100 to 0800 h). Heifers under heat stress had higher rectal temperatures and respiratory rates; lower plasma triiodothyronine and glucose; higher plasma nonesterified fatty acids and beta-hydroxybutyrate; lower intakes of dry matter, net energy for lactation, and crude protein; higher water intakes; and lower body condition scores. The decline of plasma immunoglobulins (Ig) over the final 2 wk of pregnancy was less pronounced for heifers under heat stress. For the first four milkings, colostrum of cows exposed to high air temperatures had lower mean concentrations of IgG and IgA; lower mean percentages of total protein, casein, lactalbumin, fat, and lactose; lower contents (grams per liter) of short- and medium-chain fatty acids; lower energy; lower titratable acidity; and higher pH. Thus, high air temperatures during late pregnancy and the early postpartum period markedly affected the composition of colostrum from primiparous dairy cows.