Originally Posted by genejockey

I've had something like that a couple times, but it's always early in the season, like April, on maybe the first ride on a day over 80. Weak, light-headed, vaguely nauseated. Heavy sweating. Generally only once in any given year, and no recurrence. So I've always assumed it reflects lack of heat acclimation. But if you've been riding in the heat already for some time, that doesn't make sense.