There has been considerable debate about the effectiveness of different rest interval lengths on strength gains. OBJECTIVE: The purpose of the current study was to examine the effects of different rest interval durations on upper- and lower-body strength during and after a 16 week resistance training program. Thirty-six recreationally trained men were randomly assigned to one (G1; n = 12), three (G3; n = 12) or five (G5; n = 12) minutes rest interval groups. Each group performed the same program in a nonlinear periodized training model. Maximal strength was assessed at baseline, mid-point (eight weeks) and post-training (16 weeks) for the bench press and leg press exercises. For the bench press, significant increases were demonstrated within G3 and G5 at eight weeks and 16 weeks versus baseline (p < .05). Additionally, G5 (98.2 ± 3.7 kg) was significantly stronger than G1 (92.5 ± 3.8 kg) at 16 weeks (p < .05). For the leg press, significant increases were demonstrated within all groups at eight weeks and 16 weeks versus baseline (p < .05). Additionally, there were significant differences between groups at eight weeks [i.e. G5 (290.8 ± 23.5 kg) significantly stronger than G1 (251.0 ± 15.8 kg); p < .01] and 16 weeks [i.e. G3 (305.0 ± 23.9 kg) and G5 (321.7 ± 21.7 kg) significantly stronger than G1 (276.7 ± 10.7 kg); p < .05]. The findings of the current study indicate that longer rest intervals may result in significantly greater increases in upper and lower body strength after the early weeks of training, when compared to shorter rest intervals. Shorter rest intervals can be effective for strength increases in less trained muscles or exercises; this may apply to advanced athletes following a layoff or novice athletes beginning a resistance training program. Longer rest intervals (up to five minutes) are best applied in highly trained muscles and exercises and the window for adaptation narrows.