To extend previous research on the effects of ionizing radiation on learning, dose-effect data with 60Co gamma-rays were collected for individual rats responding under a repeated-acquisition procedure. Under this procedure, subjects acquired a different three-response chain each session by responding (nose push) on one of three transilluminated response keys in the presence of each of three sequentially ordered colors. The response chain was maintained under a second-order fixed ratio (FR) 2 schedule of food presentation. An error produced a 5-s timeout but did not reset the three-response chain. Acquisition of each response chain was defined by a decrease in errors as the session progressed (i.e., within-session error reduction). Each session ended after 200 reinforcements or 90 min, whichever occurred first. When day-to-day acquisition for all four subjects reached a steady state, the effects of three or four doses of gamma-rays were assessed. In general, radiation doses of 1, 3, 4.5, and 8 Gy of gamma radiation delivered at a dose rate of 2.5 Gy/min produced a dose-dependent decrease in the overall response rate for 24-72 h after exposure in all four subjects. Radiation exposure also produced an increase in percent errors but only at doses that substantially decreased overall rate of responding. Unlike the effects on response rate, which were relatively consistent over a 72-h period, the effects on accuracy were greater at 72 h than at 24 h in three of four subjects.(ABSTRACT TRUNCATED AT 250 WORDS)