Native vegetation in northeastern Mexico is mainly composed of shrubs and small trees, which are commonly selected by white-tailed deer. The aim of the study was to determine, seasonally, the in vitro fermentation profiles of Croton cortesianus and Leucophyllum frutescens. Foliage was sampled from summer 2004 to spring 2005 for a total of 4 consecutive seasons in Los Ramones in the state of Nuevo Leon, Mexico. In vitro gas production was recorded at 3, 6, 9, 12, 24, 48, 72 and 96 h. As inoculum, rumen fluid from fistulated sheep was utilized. Kinetic parameters such as the asymptotic gas production (B), rate of gas production (k) and lag phase (L) were estimated by the exponential model G = b × (1 − e^-k(t-L)). Microbial protein synthesis, ME content and in vitro organic matter digestibility (IVOMD) were also evaluated. Data were analyzed using ANOVA for a completely randomized design. The asymptotic gas production (B) ranged from 151 mL/g DM in L. frutescens in spring to 220 mL/g DM in C. cortesianus in winter. The rate of gas production (c) was the lowest in C. cortesianus in summer (0.050%/h) while the highest (0.082%/h) in L. frutescens collected in winter. Values regarding lag phase (L) ranged from 0.43 to 22.06 h; in general, this variable was superior in C. cortesianus during summer, autumn and winter (2.06, 1.60 and 1.23 respectively). Microbial protein synthesis measured as purines was similar among shrubs and seasons. Mean values ranged from 4.04 in to 7.80 µmol in C. cortesianus in spring and winter. Estimates of ME were highest in C. cortesianus among seasons (summer 1.73; autumn, spring 1.70 and winter 1.66 Mcal/kg DM). The same scenario was registered with the IVOMD content where values varied from 55 to 84%. Highest digestibility values were registered in C. cortesianus. Data suggest that C. cortesianus represents a better feed reserve for white-tailed deer in the semiarid regions of northeastern Mexico.