Previous studies of inbred rats have shown that Brown Norway (BN) rats had more daily paradoxical sleep (PS) than Lewis (L) rats, while F1 progeny had intermediate amounts, suggesting codominant or polygenic transmission. Amount of PS and the induction of PS episodes may be under separate genetic control. Earlier work had shown that five-minute exposures to cage lights-off every half-hour can trigger PS in outbred albino strains. To explore the genetic controls for PS induction, PS triggering by dark pulse stimulation was examined in L and BN rats. L rats showed a five-fold increase in PS during dark pulse stimulation. Although, as in the earlier study, BN rats had more total daily PS than L rats, they exhibited no dark pulse triggering of PS. Thus L and BN rats show significant strain differences in two independent parameters of PS, and may be a useful model for studying genetic and neurologic factors which regulate PS.