The nephropathy induced by mercuric chloride was assessed in unilaterally nephrectomized (NPX) and sham-operated (SO) rats using histological and urinalysis techniques. This assessment was carried out in order to test whether or not rats are more susceptible to the nephrotoxic effects of mercuric chloride after unilateral nephrectomy and a period allowing for compensatory renal growth. Twelve days after surgery both NPX and SO rats were given a single 1.5, 2.0 or 2.5 mumol/kg dose of mercuric chloride (i.v.). Twenty-four hours after the 1.5 or 2.0 mumol/kg dose of mercuric chloride was administered, cellular and tubular necrosis in the pars recta segments of proximal tubules in the outer medulla was more severe in NPX rats than in SO rats. Moreover, the urinary excretion of a number of cellular enzymes (e.g. lactate dehydrogenase) and plasma solutes (e.g. albumin) was greater in NPX rats than in SO rats. At the 2.5 mumol/kg dose of mercuric chloride, renal tubular damage was quite extensive in both groups of rats; to such an extent that possible differences in renal tubular damage between the NPX and SO rats could not be determined histologically. However, the urinary excretion of alanine aminopeptidase was greater in the NPX rats than in the SO rats. Therefore, based on the aforementioned findings, rats that have undergone and adapted to a reduction in renal mass (i.e. unilateral nephrectomy) appear to be more vulnerable to the nephrotoxic effects of mercuric chloride than rats with two normal kidneys.