The effects of high and fluctuating pressure up to 220 bar on microbial growth and activity were determined in a pilot-scale water hydraulic system. An increase in the pipeline pressure from 70 to 220 bar decreased the total and the viable cell number in the pressure medium from 2.2(+/-0.5)x10(5) to 4.9(+/-1.5)x10(4) cells/ml and from 5.7(+/-2.8)x10(4) to 1.3(+/-0.7)x10(4) cfu/ml, respectively. Microbial attachment in the non-pressurised tank of the hydraulic system increased with increasing pipeline pressure [from 1.0(+/-0.3) to 3.8(+/-2.7)x10(5) cells/cm(2) on stainless steel]. The phosphatase. aminopeptidase and beta-glucosidase activities in the pressurised medium were between 0.02 and 1.4 mumol/lh (V-max) and decreased in response to increasing pipeline pressure. The alpha-glucosidase activity was detected only at 70 bar and the glucuronidase activity only occasionally. Based on principal component and cluster analyses, both the pressure applied and the original filling water quality affected ubstrate utilisation patterns. This study demonstrated the capability of freshwater bacteria to tolerate high and fluctuating pressure in a technical water system. Microbial survival was due to attachment and growth on the surfaces of the non-pressurised components and the nutrient flux released by cell lysis in the pressurised components. In summary, high pressures in water hydraulic systems do not prevent potential microbiologically related operational problems.