Analytical, intra-individual, and inter-individual components of variation were estimated from duplicate analyses of 11 plasma analytes in an average of 13 specimens obtained, over a two-day period, from nine patients with impaired renal function. Analytical variance was 17.3% of the total variance for sodium; less than 5% of the total variance for potassium, chloride, bicarbonate, albumin, and calcium; and less than 1% for urea, creatinine, glucose, creatine kinase (EC 2.7.3.2), and alanine aminotransferase (EC 2.6.1.2). Average intra-individual variances were of the same order as those found in healthy individuals. All analytes except glucose showed strong individuality. We postulate that, in nonacute pathological processes where new homeostatic steady states are reached, biological variations around the new set points are of the same order as those found in healthy individuals.