A 4-week oral study was conducted in male rats to characterize and compare the toxicity of four aliphatic ethers (butyl ether, BE; ethyl hexyl ether, EHxE; methyl heptyl ether, MHpE; and 1,6-dimethoxyhexane, DMH) which have been proposed as high-cetane diesel additives. Male Sprague-Dawley rats (280+/-20 g) were divided into groups of seven animals each and were administered by gavage low (2mg/kg body weight), medium (20mg/kg) or high (200mg/kg) doses of BE, EHxE, or MHpE, 5 days per week for 4 weeks. Another group of animals was administered DMH at 200mg/kg while the control group received the vehicle (corn oil at 1 ml/100g bw) only. At the end of the treatment period, relative testis weights and thymus weights were significantly decreased in the DMH group but not in animals receiving BE, EHxE, or MHpE. Microscopic examination revealed degeneration of the seminiferous tubules and reduction of sperm density in the epididymides in the DMH treatment group. Urinary creatine/creatinine ratio, a sensitive indicator of testicular damage, was markedly elevated in the DMH treated animals but not in those treated with BE, EHxE, or MHpE. In the bone marrow, DMH caused mild dyserythropoiesis and dysthrombopoiesis, while BE, EHxE, and MHpE produced mild increases in granulocytes and myelocyte/erythrocyte ratio. All four ethers at 200mg/kg caused mild histological changes in the thyroid but no significant modulation in the circulating thyroxin (T4) or triiodothyronine (T3) levels. All four ethers produced hepatic effects at 200mg/kg consisting of mild, adaptive histological changes, increased urinary ascorbic acid output, and elevation in the activities of one or more xenobiotic metabolizing enzymes (benzyloxyresorufin-O-dealkylase, UDP-glucuronosyltransferase, glutathione-S-transferases). The level of 2-methoxyacetic acid (MAA), a known testicular and developmental toxin, was significantly increased in the urine and plasma of animals treated with DMH but not in those administered the high dose BE, EHxE, or MHpE. Amomg the individual rats treated with DMH, the MAA level appeared to correlate with the severity of toxicity such as testicular and thymic weights, and urinary creatine/creatinine ratio. It is concluded that BE, EHxE, and MHpE differed from DMH in that they did not produce testicular or thymic toxicity. All four ethers at high dose caused changes to the thyroid, liver and bone marrows that were mild and adaptive in nature. MAA appeared to be the proximal toxicant in DMH treated animals but the route by which DMH is metabolized to MAA remains to be elucidated.