Two experiments were conducted to assess the relative effects of signal density and regularity on watchkeeping performance. In Experiment I, three levels of density (6,24, and 91i signals/hr.) were combined factorially with three levels of variability (coefficients of variation of 0.01, 0.10, and 1.00), and 10 Ss were assigned at random to each of the nine conditions. In Experiment II, five leveIs of density (6, 12, 24, 48, and 96 signals/hr.) The uncertainty of the critical signal in a watchkeeping task can be expected to affect performance (cr., Alluisi, 1966, pp. 206-208). Indeed, the kind and degree of uncertainty define, at least in part, the difficulty of the watchkeeping task.In one of the earliest of watchkeeping studies, Mackworth (1950, pp. 53-63) reported that the deterioration in correct signal detections which typically occurred in his synthetic radar task could be delayed by increasing the brightness of the critical signals. This suggests that watchkeeping performance is a decreasing function of stimulus uncertainty (uncertainty concerning the nature of the critical signal). That is to say. as the signal-tonoise ratio of the critical signal is increased, performance efficiency also can be expected to increase. Although there have been few, if any, functional studies of stimulus uncertainty, it has been well established that this kind of uncertainty is likely to have some effect on watchkeeping performance (e.g., Adams, 1956;Bartlett et al , 1955;Jenkins, 1953Jenkins, , 1958.Spatial uncertainty(uncertainty concerning where the signal will appear) has been found to affect performance in visual watchkeeping tasks. Mackworth (1950, pp, 58-59), again using his synthetic radar task, found a progressive decline in the incidence of missed signals with rising spatial expectancy, or decreasing spatial uncertainty, and essentially similar results have been reported by other investigators (Deese & Ormond. 1953;Bartlett et al, 1955; Nicely & Miller,1957; Adams & Boulter, 1964).The third major source of uncertainty has also been studied; this is temporal uncertainty, or uncertainty concerning when a critical signal will occur. Both Ditchburn (1943) and Mackworth (1950) report that changes in the temporal intervals between signals produced associated changes in the proportions of signals missed. It appears from the results of later studies that changes either in signal frequency or in signal regularity may affect performance efficiency in watchkeeping tasks (e.g., Baker, 1959a, b;Bartlett et al , 1955;Broadbent, 1958;Deese, 1955;Deese & Ormond, 1953;Jenkins, 1958;Smith, 1961). These findings suggest that a psychophysical approach could be taken in studying watchkeeping performance, provided, of course, that the physical aspects of the uncertainty of the critical signal can be quantified and controlled. Such an approach has been taken in the present study in an attempt to determine the psychophysical relations between watchkeeping performance on the one hand, and the temporal uncertainty of the critical signa...