The extracellular signal-regulated kinases (ERKs) are members of the mitogen-activated protein kinase (MAPK) superfamily of enzymes and have recently garnered considerable attention in the field of learning and memory. ERK activation has been shown to be required for the induction of long-term potentiation (LTP) in the rat hippocampus and for the formation of associative and spatial memories in both the rat and the mouse. However, the individual roles for the two isoforms of ERK have yet to be deciphered. To investigate the specific contribution of the ERK1 (p44) isoform of MAPK to mammalian learning, we performed a general behavioral and physiological characterization of mice lacking the ERK1 gene. The ERK1-null animals demonstrated significantly higher levels of activity in the open field test. However, we observed no other discernible deficits in the ERK1 knockout mice in our behavioral testing. Specifically, no differences were observed in the acquisition or retention (24 h and 2 wk after training) of either contextual or cue fear conditioning between the ERK1^−/− and their wild-type littermate controls. In addition, no learning phenotype was observed in the passive avoidance test. When hippocampal slices were analyzed, we found no deficits in baseline synaptic transmission or in tetanus-induced LTP in hippocampal area CA1. We found no apparent compensatory changes in the expression of ERK2 (p42 MAPK). We conclude that hippocampus- and amygdala-dependent emotional learning does not depend critically on the activity of ERK1.