1. The interaction between the center and surround mechanisms of a var
iety of rabbit retinal ganglion cell classes was examined in extracell
ular single-unit recordings in an isolated eyecup preparation. Ganglio
n cell classes studied included ON and OFF brisk sustained and transie
nt, ON and OFF sluggish sustained and transient, ON-OFF and ON directi
onally selective, orientationally selective, and large field units. Th
e surround effects observed were qualitatively similar in all these ga
nglion cell classes. 2. The average reponse-versus-contrast functions
for stimuli within the ganglion cells' receptive-field centers were re
latively linear between threshold and saturation for all ganglion cell
classes examined. The major effect of surround stimulation on the cen
ter response-versus-contrast function was a reduction in the slope of
the linear portion of the curve, rather than a downward, parallel shif
t of the function. Stimulation of the surround had no systematically s
ignificant effect on the contrast threshold for the center spot, and,
when it did have a significant effect, it sometimes decreased, rather
than increased the magnitude of the threshold. 3. Step changes in surr
ound contrast were most effective when they were made simultaneously w
ith step changes in the center; surround inhibition decreased signific
antly when it preceded stimulation of the center by >100 ms and was ge
nerally ineffective when preceding the center by >500 ms. The decrease
in the inhibitory effect of surround stimulation was a monotonic func
tion of delay between 0 and 500 ms. 4. Stimulation of the surround by
step changes in the contrast of a sine-wave grating annulus produced q
ualitatively similar re suits to those obtained for pure luminance mod
ulations. This suggests that the surround mechanism observed in these
experiments was not due to pure luminance adaptation within the surrou
nd. The inhibitory effect of sine-wave gratings in the surround decrea
sed monotonically as a function of spatial frequency. 5. Stimulation w
ith a spot and an annulus that were both entirely within the ganglion
cell's excitatory receptive-held center typically yielded nonadditive
summation at contrasts whose linear sum of responses were below satura
tion. The effect of an annulus within the receptive-field center on re
sponses elicited by a central spot quantitatively resembled the inhibi
tion elicited from annuli in the inhibitory surround, after the excita
tory center response due to the annulus was taken into account. These
results suggest that the inhibition elicited from the surrounds of the
ganglion cells in these experiments extended into their receptive-fie
ld centers. 6. The strength of inhibition elicited from the surround d
eclined with reductions in the background level, both for a given modu
lation depth, and for the same absolute modulation amplitude. At the l
owest backgrounds used, there was no apparent inhibition of center res
ponses by the surround. 7. Particularly for transient ganglion cells,
the nature of the interaction between surround and center mechanisms i
s generally consistent with nonlinear, division-like models for center
-surround interaction, such as contrast gain control, but not with lin
ear models in which the surround is subtractively antagonistic to the
center.