A 6 mon (6 hr/day, 5 days/week) inhalation toxicity study was conducted with cyanogen gas using male rhesus monkeys (Macacca mulatta) and male albino rats (Charles River Strain) as experimental animals. Fifteen monkeys and 90 rats were divided into three groups of 5 monkeys and 30 rats. One group, the Controls, was not exposed to the test material; the other two groups were exposed to either 11 ppm or 25 ppm cyanogen. At the outset of exposures, there was a doubling of the rate of responding on a variable interval 2.9 min schedule of reinforcement in monkeys exposed to 25 ppm cyanogen, and increases were also seen in the monkeys receiving 11 ppm exposures; the increases were transitory as the rate returned to control levels before exposures were terminated. At the end of the 6 mon exposure, there were no effects in hematologic or clinical chemistry parameters attributable to the inhalation exposure to cyanogen. The electrocardiograms, and gross pathologic and histopathologic examinations of test animals were normal when compared with the Control animals. Total lung moisture content was significantly lower in monkeys exposed to either 11 ppm or 25 ppm cyanogen than in Control animals. Body weights were significantly lower in rats exposed to 25 ppm than in Controls. The results suggest that subchronic 25 ppm cyanogen exposures are marginally toxic, but the evidence on 11 ppm does not support a similar conclusion.