Male and female Sprague-Dawley rats were administered drinking water containing humic acids either non-disinfected or following ozonation (O3) or ozonation/chlorination (O3/CL2) for 90 consecutive days. Test animals drank either of two concentrations of humic acids, 0.25 and 1.0g/L total organic carbon (TOC), while controls received phosphate-buffered, distilled water. No consistent significant treatment-related effects were observed in body weight gain, organ weights, food or water consumption, or hematological and clinical chemistry parameters. No target organs were identified from the histopathological examination of the tissues. The most significant observation, an increase in liver to body weight ratio for the male animals in the 1.0g/LO3/CL2 humic acid group, was not observed in any other group, nor was it corroborated via any biochemical measurements or histopathological analysis. (Copyright (a) 1990 American Chemical Society.)