Longitudinal halves of freshly harvested avocado fruit (Persea americana Mill. `Hass') were pretreated at 38C for 1 hour in a water bath, while the other half remained at 20C in air. Then the entire fruit was either treated from 1 to 10 minute at 50C, or held at 20C (controls). Fruit quality (daily evaluation of browning and internal quality when ripe), and pulse amplitude modulated (PAM) fluorescence measurements, were made on the skin of each fruit half 1 hour after hot water treatment (HWT), 3 hours later, and each subsequent day until ripening. The pretreated half of the fruit showed almost no development of external browning during the ripening period, while the nonpretreated halves were severely damaged by HWTs. External browning increased with longer HWT duration. Heat damage was also evident as hardening of the skin when fruit ripened, and such damage was reduced by pretreatment and increased with longer HWT duration. HWT had a rapid and marked effect on chlorophyll fluorescence (Fv/FM ratio) of avocado skin. Whereas fluorescence of control fruit remained constant over the first 5 days, in both pretreated and nonpretreated fruit, within 1 hour of HWT, the Fv/FM ratio had dropped to near minimal levels, with little further change. The value of Fv/FM 3 to 6 hours after the HWT was directly related to the duration of the HWT (P <0.0001). Although pretreatment almost eliminated browning, little effect of pretreatment could be detected in the Fv/FM ratio. There was a strong negative correlation (r = 0.93, P < 0.0001) between external browning and Fv/FM for nonpretreated fruit, but this correlation was not significant for pretreated fruit. We conclude that chlorophyll fluorescence clearly reflects effects of heat on the photosynthetic systems in avocado fruit, but does not detect the alleviation of heat damage by pretreatments.