The effect of increase ambient temperature and sunlight on the temperatures of the rabbit lens and posterior chamber (PC) aqueous humour was measured by needle thermistor probes while the rectal temperature was monitored. Exposure of rabbits to sunlight (35 degrees-42 degrees C), in New Haven, Connecticut, USA, resulted in significant temperature increases in PC (4.3 degrees C), lens (3.2 degrees C), and rectum (2.3 degrees C). Returning animals to the shade resulted in a progressive decrease in the temperatures of the PC or lens in the tested eye, but repeating exposure to sunlight resulted in significant increases of the baseline (PC) temperature (increase 2.68 degrees C) of the second eye. Exposure of rabbits to sunlight at 49 degrees C in Chandigarh, India, resulted in increased PC temperature of 4.48 degrees C after 9 minutes. Increased PC and lens temperatures after exposure to sunlight are due both to an ambient temperature effect through the cornea and to increased body temperature. In dry and hot tropical areas of the world temperature increases in the lens after exposure to sunlight may initiate or accelerate the formation of senile cataracts.