Interleukin-4 (IL-4) and interleukin-13 (IL-13) are Th2 cytokines involved in various immune responses and only a few reports examine skeletal muscle and the possible role of Th2 cytokines in myoblast fusion and myotube maturation. We hypothesized that IL-4, IL-13 and their receptors are involved in skeletal muscle adaptations to strength training. We investigated the effects of 6 weeks of strength training with different muscle load on IL-4, IL-13, IL-4Ralpha and IL-13Ralpha1 expression in skeletal muscle: training with maximal contractions (Max group, n = 12) and training with lower load combined with ballistic and stretch-shortening contractions (Combi group, n = 12). Muscle samples were taken from triceps brachii before and after the 6 weeks of training. mRNA levels of IL-4, IL-13, IL-4Ralpha and IL-13Ralpha1 expression were determined by real-time PCR and IL-4 protein expression by ELISA. We demonstrate for the first time that IL-4, IL-13, IL-4Ralpha and IL-13Ralpha1 are expressed in skeletal muscle in vivo and are up-regulated after strength training. Higher training load in Max leads to higher cytokine responses, which was significant for IL-4Ralpha though IL-4 protein expression did not change significantly. Although the exact mechanisms have yet to be established, and these results may be preliminary, we speculate that IL-4, IL-13, IL-4Ralpha and IL-13Ralpha1 are involved in muscle hypertrophy, MHC-transformation and anti-inflammatory damage control that occur during strength training.