The objective of this study was to determine the effect of a subacute ruminal acidosis (SARA) challenge on CH₄ emission rate from lactating cows. We hypothesized that low ruminal pH resulting from SARA would reduce CH₄ emission substantially especially in cows fed a low forage diet. Eleven ruminally cannulated Holstein cows (711 ± 59 kg BW; 103 ± 59 DIM) were randomly assigned to either a 45 or 67% forage diet (DM basis) in a complete randomized design. After 2-wks of adaptation to a GreenFeed CH₄ measuring unit, 4 cows on each treatment were selected to undergo the SARA induction protocol, which included 4 periods: baseline (4 d), feed restriction (50%; 1 d), challenge (TMR + wheat-barley pellet at 20% of baseline DMI; 2 d), and recovery (normal dietary treatments feeding; 2 d). Rumen pH was recorded every 5 min for 7 of the 9 d of protocol using wireless pH meters. Methane (g/h) was measured at 12 p.m., 5 p.m., 9 p.m., and 6 a.m. during the induction protocol. Overall, milk yield (40.4 ± 3.6 kg/d, P = 0.16), DMI (23.1 ± 1.8 kg/d, P = 0.26), mean ruminal pH (6.08 ± 0.15, P = 0.84), nadir ruminal pH (5.49 ± 0.15, P = 0.31), hours below pH 5.6 (3.2 ± 1.78 h/d, P = 0.31) and CH₄ emission rate (12.2 ± 1.0 g/h, P = 0.29) were not affected by dietary treatments. There was no treatment by period interaction but a significant period effect for reported variables. Compared with baseline (27.3 kg/d) DMI was not different (P = 0.82) during the challenge (26.1 kg/d) but was lower (P = 0.05) during recovery (25.2 kg/d). Milk yield dropped (P < 0.01) from 45.5 kg/d during baseline to 32.5 kg/d during challenge and returned almost to baseline during recovery (41.2 kg/d, P = 0.10). Methane emission (g/h) rate were 14 and 26% lower during challenge (11.7, P = 0.03) and recovery (10.1, P < 0.01) compared with baseline (13.6). During recovery, CH₄/DMI (g/kg) was lower (9.7, P = 0.05) compared with baseline (12.2). Hours below pH 5.6 increased from 1.5 to 6.4/d from baseline to challenge (P < 0.01). In this study, SARA reduced CH₄ emission but there was a complex temporal relationship between rumen pH and CH₄ emission during and after an acidosis episode.