Willie J. Stephens appeals from his conviction of two counts of violating the Georgia Controlled Substances Act following the denial of his motion for a new trial.

1. In the absence of a timely written request, the trial court did not err in failing to charge the jury that the defendant had a constitutional right not to testify and that no inference could be made as a result of his failure to testify on his own behalf. Woodard v. State, 234 Ga. 901 (218 SE2d 629) (1975).

2. Assuming arguendo that appellant's objection to the testimony of a witness as to the purpose of undercover agents was properly made, the trial judge did not err in permitting the witness to give reasons, circumstances and conditions surrounding the conduct of the drug investigation. Rozier v. State, 124 Ga. App. 481 (184 SE2d 203) (1971); Taylor v. State, 135 Ga. 622 (4) (70 SE 237) (1910); and James v. State, 223 Ga. 677, 684 (7) (157 SE2d 471) (1967).