In pearlitic steels, hydrogen trapping at interphase boundaries may induce hydrogen- cracking in the absence of external applied stress such as in blistering and Hydrogen Induced Cracking. However, in low alloy steels containing a much lower density of such trapping sites and/or when hydrogen activity is reduced, most instances of hydrogen-induced cracking involve strong interactions between local plasticity and hydrogen effects. We investigated the effects of these variables on the crack growth rate in two quenched and tempered high strength low alloy (HSLA) steels. Slow strain rate tensile and low-amplitude cyclic tests were conducted on micro-notched specimen in conditions of variable hydrogen activity. The results indicate a strong dependence of crack growth rate with the tempering on the one hand and both the notch-tip plastic zone and hydrogen activity on the other. At low activity, dynamic strain seems to be the major factor affecting hydrogen embrittlement, while trapping effects appear dominant at higher hydrogen activity.