Conditioned taste aversion was demonstrated to result from a US-CS, or "backward," conditioning procedure using a 75-mg/kg cyclophosphamide injection as the US. The magnitude and resistance to extinction of the conditioned taste aversion was found to decrease monotonically when the US·CS lSI was systematically varied from 1.0 min to 4 h. In this respect and in the inconclusive nature of the signs of sickness following US administration, cyclophosphamide appears to act similarly to ionizing radiation in conditioning taste aversions. It was argued that these results reflect truly associative, not pseudoconditioned, avoidance responses.