Table 1.

Incidences of neoplasms and nonneoplastic lesions of the kidney in male F344/N rats exposed to methyl isobutyl ketone by inhalation for two years.

	0 ppm	450 ppm	900 ppm	1800 ppm
Number Examined Microscopically	50	50	50	50
Nephropathy
Single Sections	42a (2.0)b	45 (2.6)▲	47 (2.4)▲	50* (3.1▲
Papilla Mineralization
Single Sections	1 (1.0)	6* (1.2)	22** (1.6)	29** (1.5)
Pelvis Transitional Epithelium Hyperplasia
Single Sections	1 (1.0)	5 (1.8)	6* (1.2)	19** (1.4)
Renal Tubule Hyperplasia
Single Sections	1 (2.0)	11** (3.2)	3 (2.0)	18** (2.7)
Step Sections	0	3 (2.0)	4 (2.0)	6* (2.3)
Single and Step Sections (Combined)	1 (2.0)	14* (2.9)	7* (2.0)	21** (2.5)
Renal Tubule Adenomac
Single Sections	0*	0	2 (4.7)d	3 (7.3)
Step Sections	2 (4.5)*	3 (7.2)	1 (2.4)	7 (16.8)
Single and Step Sections (Combined)	2 (4.5)**	3(7.2)	3 (7.1)	10 (24.0)**
Renal Tubule Carcinomae
Single Sections	0	1 (2.4)	0	2 (4.8)
Step Sections	0	0	0	0
Single and Step Sections (Combined)	0	1 (2.4)	0	2 (4.8)
Renal Tubule Adenoma or Carcinomaf
Single Sections	0*	1 (2.4)	2 (4.7)	4 (9.7)
Step Sections	2 (4.5)*	3 (7.2)	1 (2.4)	7 (16.8)
Single and Step Sections (Combined)	2 (4.5)**	4 (9.5)	3 (7.1)	11 (26.4)**

^alesion incidence;

^baverage severity grade of lesions in affected animals: 1=minimal, 2=mild, 3=moderate, 4=marked;

^cHistorical incidence for 2-year inhalation studies with chamber controls given NTP-2000 diet (mean ± standard deviation): 3/399 (0.8% ± 1.0%); range, 0%–2%;

^dsurvival adjusted incidence (%);

^eHistorical incidence: 1/399 (0.3% ± 0.7%); range, 0%–2%;

^fHistorical incidence: 4/399 (1.0% ± 1.1%); range, 0%–2%;

^▲Increased lesion severity by Mann Whitney U test;

^*P≤0.05 by the poly-3 test (for trend if assigned to control);

^**P≤0.01