Much of what is known of the activities of polycystin-1 has been inferred from the effects of the isolated cytoplasmic COOH-terminal domain, but it is not clear whether the truncation acts like polycystin-1, as a dominant negative, or in unrelated pathways. To address this question, we have examined functional interactions between the intact and truncated forms of polycystin-1 in one cell system. In cells expressing only native polycystin-1, introduction of the truncation replicated the activity of the full-length protein. Conversely, when background levels of polycystin-1 were modestly elevated, the truncation acted as a dominant negative. Hence, the truncation acts in the polycystin pathway, but with effects that depend upon the background level of polycystin-1 expression. Our data raise the possibility that the cytoplasmic carboxyl terminus, either through cleavage products or intramolecular interactions, might feed back to modulate the activity of parent or intact polycystin-1.