Two groups of male Wistar rats were chronically implanted with small inflatable balloons at the right superior vena caval (SVC)-atrial junction. The balloons were inflated for 60 min in one-third of the rats in each group; the remainder served as noninflated controls. Extract was prepared from the right atria of the first and from the left atria of the second group and bioassayed in male Wistars to determine the content of atrial natriuretic factor (ANF). The bioassay rats received either two injections of sham extract (control) or an initial injection (I1) of sham extract followed by an injection (I2) of balloon-inflated extract (experimental). For right atrial extract, the incremental response to the experimental injection (I2/I1) was significantly less than the response to the control injection (experimental, 1.15 +/- 0.09 mueq/min, n = 14; control, 1.59 +/- 0.15 mueq/min, n = 13; P less than 0.01). However, in the case of left atrial extract, there was no such difference between the control group (1.35 +/- 0.13 mueq/min, n = 19) and the experimental group (1.25 +/- 0.11 mueq/min, n = 11; Student's t test). We conclude that right atrial distension causes release of ANF from the right atrium, but not the left, and that this release is probably locally mediated.