Free, acetyl-, medium- and long-chain acylcarnitine and total plasma carnitine concentrations were measured in eight continuous ambulatory peritoneal dialysis (CAPD) patients and eight age- and sex-matched healthy controls. Daily loss of carnitine was also quantified in both groups, by analysis of urine and dialysis fluid. Plasma total carnitine concentration in CAPD patients was not significantly different from controls (42.8 +/- 1.6 and 43.1 +/- 2.3 mumol/liter, respectively). However, the plasma free carnitine concentration of CAPD patients was significantly lower than that of controls (28.5 +/- 1.4 and 36.2 +/- 2.5 mumol/liter, respectively; P < 0.05). No difference in the daily loss of total carnitine was found between CAPD patients and controls (269.7 +/- 30.0 and 240.5 +/-33.0 mumol/liter, respectively), but the daily loss of free carnitine was significantly greater in CAPD patients (175.8 +/- 17.3 and 105.8 +/- 16.4 mumol/liter, respectively; P < 0.05). The ratio of total acylcarnitine (acetyl-, medium- and long-chain acylcarnitine) to free carnitine was significantly greater in plasma of CAPD patients than in controls (P < 0.01) and was lower in daily fluid losses (P < 0.001). These ratio differences suggests that an alteration in acyl group metabolism is occurring in CAPD patients. This may be attributable to an accumulation of medium- and long-chain acylcarnitine in liver of CAPD patients which would be exchanged for plasma free carnitine and/or to a differential loss of free and acylcarnitine across the peritoneal cavity.