Six closely related analogues of somatostatin were tested for their ability to inhibit electrically induced contractions of the mouse vas deferens. Their inhibiting activities tended to parallel their reported effects on gastric acid secretion in vivo, while no correlation to their in vitro growth hormone release-inhibiting activities was observed. It is suggested that information derived from the vas deferens assay may provide a relatively rapid and inexpensive assessment of the effects of somatostatin analogues on gastric acid secretion. The vas deferens assay was also used to test several somatostatin analogues for antagonistic activity, which was not found.