Thermotolerant mutants of wheat cv. Guardian were isolated by selecting survivors from 5-d-old seedlings of M2 populations exposed to 47 degrees C for 90 min. Progeny testing, using triphenyl tetrazolium chloride reduction as a measure of tissue viability, following heat stress treatment for 120 min at each of three temperatures (32, 38 and 50 degrees C), confirmed the thermotolerant nature of seedlings of 13 mutants. Mutants were isolated at frequencies of 0.1% and 0.2% following the use of sodium azide and ethyl methanesulphonate, respectively. The relative thermotolerance of ten of the mutants and 'Guardian' was then tested by exposing plants to heat stresses of 38 degrees C for 6 h in every 24 h for five successive days at one of four growth stages between seedling and anthesis. Pmax (light-saturated net photosynthetic rate) and chlorophyll content were compared in stressed and unstressed plants. The Pmax of 'Guardian' was depressed by at least 23% by heat stress at each stage; this inhibition was least at ear emergence and greatest at anthesis, the latter being associated with reduced sink size as a result of lowered seed set. The stress-induced inhibition of Pmax in 'Guardian' plants at anthesis had not recovered 3 d after removal of the stress. Mutant lines exhibited different developmental profiles of Pmax thermostability. Mutant tht (thermotolerant) 10, for example, exhibited partial thermostability at each growth stage tested while the Pmax of mutant tht 2 was completely unaffected by heat stress at second node and ear emergence, but was as inhibited as that of 'Guardian' at anthesis; heat stress applied at anthesis in tht 2, but not tht 10, was associated with reduced seed set. Generally, the inhibitory effect of heat stress on Pmax in the mutants was reflected in declines in chlorophyll content. The ten mutants were grouped into nine categories, on the basis of thermotolerance characteristics.