Maternal hyperthermia of short duration had significant short and long term effects on the developing guinea-pig eye at embryonic days 19, 21 and 23. A marked regionalisation of the heat-induced effects was observed in the developing retinae, but the lens defects were most obvious following heat at E21 and E23. Bilateral anterior pole cataracts, bipolar zonular cataracts and microphthalmia have been produced following maternal heat stress. It is suggested that these are determined by the cellular events initiated in the developing eye following the teratogenic insult.