Many virtual reality systems have a distributed structure for certain purposes such as more computational power, tele-presence, collaboration, and portability. However, network delays are inevitable in the distributed structure and often make sensory information delivered behind time to the user. In the literature, the effect of network delays on the quality of virtual environments has been considered mostly with respect to task performances. In this paper, we pay attention to whether perceptual artifacts caused by network delays are perceptible by the user, which is a more stringent criterion than the degradation of task performance. We examined minimum perceptible visual and/or haptic rendering delays by measuring their discrimination thresholds between normal and delayed virtual environments with and without a task, and report the results in this paper. We also provide a simple guideline for determining whether some active delay compensation algorithms are required in a distributed virtual reality system by comparing representative network delays to the measured discrimination thresholds.