Runyon was convicted on three counts of possessing unregistered firearms in violation of 26 U.S.C. §§ 5861(d) and 5871. The sole contention raised on appeal is whether the trial court committed plain error in failing to give an unrequested insanity instruction to the jury. In our judgment, the court did not plainly err. The charge of possession of unregistered firearms requires only that the defendant be shown to have possessed an item that he knew to be a firearm. Morgan v. United States, 564 F.2d 803, 805 (8th Cir. 1977). Runyon introduced testimony by a psychiatrist in an attempt to show he did not have that knowledge, and the trial judge instructed the jury on that issue. In light of Runyon’s failure to pursue an insanity defense at trial or to request an instruction thereon, plain error was not shown by the court’s failure to instruct the jury on that defense. See United States v. DiBenedetto, 542 F.2d 490, 494 (8th Cir. 1976).