Some physiological parameters were measured in adult rainbow trout during a 10-day exposure to 180 pg Al,,,,, I-' in acid water (pH 4.7) with or without humic substances (10 mg I-'). The fish were acclimatized to pH 5.0 for 7 days prior to the experimental treatments.Chemical analyses revealed that, in the presence of human substances, 7&80% of the A1 was organic bound, while in the absence of humic substances most of the Al(98%) occurred in the inorganic form.A1 bound to humic substances (13G150 pg I-') did not alter the plasma NaC1-concentration, nor the haematocrit value, of rainbow trout during an exposure period of 10 days. This contrasts with the high death rate obtained within 2-3 days when most of the A1 (175 pg I-') was in the inorganic form. The lethality was accompanied by a 25% decrease in the plasmaconcentration of NaCl and a doubling of the haematocrit value. Bulk analysis revealed that when the metal was present in inorganic forms the total Al content of the gills (75 wg A1 g-' wet weight) was I5 times higher than when it was present as bound to the humic substances. These experiments showed that the accumulation of A1 at the gills was accompanied by physiological disturbances, both being a function of the chemical speciation of Al.

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