Previous investigators have suggested that presence or absence of the CS during the interstimulus interval (ISI) may significantly affect the amount of UCR diminution taking place during classical conditioning. 4 groups of undergraduate Ss each received 16 presentations of paired or unpaired light and shock with a CS length of 1 or 5 sec. and an ISI of 5 sec., followed by 4 UCS-along test trials. The dependent variable was GSR magnitude measured within 6 sec. of stimulus onset. Significant UCR diminution during acquisition and recovery during test trials were demonstrated by both paired groups. The CS duration did not significantly affect the amount of diminution or recovery taking place. If a trace-delay difference is a necessary deduction from the conditioned inhibition explanation of UCR diminution phenomena, the present data do not support such an explanation.