Artificially mediated linear (methylviologen) and cyclic (phenazine methosulfate) electron transport induced zeaxanthin-dependent and independent (constitutive) nonphotochemical quenching in osmotically shocked chloroplasts of pea (Pisum sativum L. cv Oregon). Nonphotochemical quenching was quantitated as Stern-Volmer quenching (SV(N)) calculated as (F(m)/F'(m))-1 where F(m) is the fluorescence intensity with all PSII reaction centers closed in a nonenergized, dark-adapted state and F'(m) is the fluorescence intensity with all PSII reaction centers closed in an energized state. Reversal of quenching by nigericin and electron-transport inhibitors showed that both quenching types were energy-dependent SV(N). Under light-induced saturating DeltapH, constitutive-SV(N) reached steady-state in about 1 minute whereas zeaxanthin-SV(N) continued to develop for several minutes in parallel with the slow kinetics of violaxanthin deepoxidation. SV(N) above the constitutive level and relative zeaxanthin concentration showed high linear correlations at steady-state and during induction. Furthermore, F(o) quenching, also treated as Stern-Volmer quenching (SV(O)) and calculated as (F(o)/F'(o))-1, showed high correlation with zeaxanthin and consequently with SV(N) (F(o) and F'(o) are fluorescence intensities with all PSII reaction centers in nonenergized and energized states, respectively). These results support the view that zeaxanthin increases SV(N) above the constitutive level in a concentration-dependent manner and that zeaxanthin-dependent SV(N) occurs in the pigment bed. Preforming zeaxanthin increased the rate and extent of SV(N), indicating that slow events other than the amount of zeaxanthin also affect final zeaxanthin-SV(N) expression. The redox state of the primary electron acceptor of photosystem II did not appear to determine SV(N). Antimycin, when added while chloroplasts were in a dark-adapted or nonenergized state, inhibited both zeaxanthin-SV(N) and constitutive-SV(N) induced by linear and cyclic electron transport. These similarities, including possible constitutive F(o) quenching, suggest that zeaxanthin-dependent and constitutive SV(N) are mechanistically related.