The discrimination ability of filled (1 kHz pure tones) and empty (gaps bounded by two clicks) intervals ranging from 50 msec to 12 sec was investigated. The relative accuracy of discrimination showed U-shaped curves for both intervals. A model, comprising a logarithmic conversion process of physical time to sensory quantity, a short-term memory and two internal noise sources, was proposed. It was found that the model was adequate for intervals shorter than 1 or 2 sec. Participation of other coding mechanisms was implied for time intervals beyond this range.