This study examined the effect of mothers' exposure to the 1957 A2/Singapore influenza virus in the first, second, and third trimesters of pregnancy on the electrodermal activity of their 15-year-old offspring. The epidemic occurred during cold months of the year. The results show that maternal viral (and cold) exposure during all trimesters of pregnancy produced a lower frequency of nonspecific electrodermal responses and diminished responsivity to 75-db and 90-db tones as compared with the offspring of mothers who were not exposed. However, there was a critical effect of second trimester exposure as indicated by an increase in frequency of nonresponses to orienting tones. The findings are compared with those from an earlier study from Mauritius involving 3-year-old children in which the effects of exposure to the 1968 A2/Hong Kong virus were examined and in which the effect of cold exposure could be examined separately because viral exposure occurred during the hot months.