The optimum CS-US interval for classical conditioning in an infrahuman primate was investigated in 54 rhesus monkeys using CS-US intervals of .5, 1, 2, and 4 sec. The CS was an increase in illumination, and the US was a dc shock lasting .1 sec. Only the 2-sec. condition gave significantly more CRs than the control condition, which consisted of CS-only and US-only trials in random order. The results are discussed in relation to recent theoretical statements. (PsycINFO Database Record (c) 2006 APA, all rights reserved)