Levels of required margin are shown to be positively related to autocorrelations in stock returns, a result which implies that the probability of nontrading increases when margin levels increase. Frequencies of stock-price reversals are studied to determine the effect of margin requirements on participation by information traders. If price reversals are negatively (positively) related to high levels of margin, then relative proportions of information traders increase (decrease). We find that reversals occur more. frequently prior to the regulation of margin in 1934. This date coincides with a general increase in the level of margins. Our logit specifications indicate that reversal probabilities are conditional on the level of margin. Controls for the effects of time and the introduction of regulation do not alter this conclusion. The results suggest that margin costs reduce stock market participation with a lesser effect on information based trading.