Physiological experiments on the properties of temporal summation in single cells of the peripheral visual system of Limulus have shown that the type of temporal summation and the value of the critical duration depend on the way in which the investigator arbitrarily chooses to analyze the same physiological response. The behavioral expression of temporal summation should therefore depend upon the way in which the central nervous system actually analyzes responses transmitted by peripheral structures as well as upon the nature of these peripheral responses. Accordingly, the present two behavioral experiments investigated the analysis of temporal summation carried out by the central nervous system of Limulus by investigating the psychophysics of temporal summation mediated by the ventral eye. Supersummation occurred in both psychophysical experiments up to critical durations near 4.5 sec, which was approximately the same as the reaction times or latencies of the behavioral responses at threshold. The degree of supersummation was extreme; a fourfold change in stimulus duration required an almost 40,000-fold change in stimulus intensity to remain at threshold. Even though some features of physiologically recorded receptor potentials have critical durations less than 10 msec, these findings suggest that the central nervous system of Limulus can integrate peripheral signals over intervals of many seconds. Since the source of the supersummation is the nonlinearity of the receptor transfer function at high levels, these results also indicate that a behavior at threshold can be mediated by a strong neural response.