Uptake, biotransformation, and elimination rates were determined for pentachlorophenol (PCP), methyl parathion (MP), fluoranthene (FU), and 2,2',4,4',5,5'-hexachlorobiphenyl (HCBP) using juvenile Hyalella azteca under water-only exposures. A two-compartment model that included biotransformation described the kinetics for each chemical. The uptake clearance coefficients (k(u)) were 25.7 +/- 2.9, 11.5 +/- 1.1, 184.4 +/- 9.3, and 251.7 +/- 9.0 (ml g(-1) h(-1)) for PCP, MP, FU, and HCBP, respectively. The elimination rate constant of the parent compound (k(ep)) for MP was almost an order of magnitude faster (0.403 +/- 0.070 h(-1)) than for PCP and FU (0.061 +/- 0.034 and 0.040 +/- 0.008 h(-1)). The elimination rate constants for FU and PCP metabolites (k(em)) were similar to the parent compound elimination 0.040 +/- 0.005 h(-1) and 0.076 +/- 0.012 h(-1), respectively. For MP, the metabolites were excreted much more slowly than the parent compound (0.021 +/- 0.001 h(-1)). For PCP, FU, and MP whose metabolites were measured, the biological half-life (t(1/2p)) of the parent compound was shorter than the half-life for metabolites (t(1/2m)) because the rate is driven both by elimination and biotransformation processes. Thus, H. azteca is capable of metabolizing compounds with varying chemical structures and modes of toxic action, which may complicate interpretation of toxicity and bioaccumulation results. This finding improves our understanding of H. azteca as a test organism, because most biomonitoring activities do not account for biotransformation and some metabolites can contribute significantly to the noted toxicity.