Response surface models were developed by multiple regression techniques to describe daily rhythms of plasma thyroid hormones in young chickens during acclimation to a wide range of ambient temperatures (Ta). Seventy-two Leghorn cockerels were raised in six environmental chambers under a 12L:12D light/dark cycle. Beginning at 21 days of age, two replicate chambers provided Ta treatments of 10, 24, or 38 degrees. During Day 6 of acclimation, blood samples were obtained at 0800, 1200, 1600, 2000, 2400, and 0400 hr for thyroid hormone analyses. Plasma triiodothyronine (T3) levels showed a convex response to the experimental region (10 to 38 degrees, and 0800 to 0400 hr under a 12L:12D light/dark cycle), whereas the plasma thyroxine (T4) response surface was concave. In particular, heat exposure (38 degrees) depressed (P less than .01) plasma T3 levels (2.8 ng/ml) below those of birds held at 24 degrees (4.5 ng/ml), whereas elevated (p less than 0.05) T3 concentrations were found in the 10 degrees treatment (5.0 ng/ml). Plasma T3 levels rose gradually during photophase and slowly declined during scotophase. Plasma T4 concentrations followed an opposite pattern with the daily zenith in scotophase and the nadir during photophase. Plasma T4 levels were sharply elevated in birds maintained at 38 degrees, particularly during scotophase. Although Ta did not change the daily amplitude or circadian frequency of the plasma T3 rhythm, an increased frequency or ultradian rhythm of plasma T4 was observed in cold-acclimated birds. The counter-responses of T3 and T4 to heat exposure and phase of the light/dark cycle were best seen in the molar ratio of plasma T3/T4 levels. The average daily T3/T4 molar ratio for heat-exposed birds was 0.16 while the average T3/T4 ratio was higher (P less than 0.01) for the 10 degrees (0.46) and 24 degrees (0.41) treatments. The opposing responses of plasma T3 and T4 levels to a wide range of Ta and phase of the light/dark cycle support a major role for circulating T3 in metabolic adjustments that chickens make to changes in the photothermal environment.