We simulate a series of daily returns from intraday price movements initiated by microstructure elements. Significant evidence is found that daily returns and daily return volatility exhibit first order autocorrelation, but trading volume and daily return volatility are not correlated, while intraday volatility is. We also consider GARCH effects in daily return series and show that estimates using daily returns are biased from the influence of the level of prices. Using daily price changes instead, we find evidence of a significant GARCH component. These results suggest that microstructure elements have a considerable influence on the return generating process.Keywords: inventory control, bid-ask spread, volatility dynamics, GARCH JEL Classification: C22, G10It is a well known fact from a large number of empirical investigations that financial return series exhibit specific patterns, e.g. positive autocorrelations in volatility. Many of these patterns have been modeled using GARCH processes as introduced by Engle (1982) and Bollerslev (1986) and developed further by many other authors. Bollerslev et al. (1992) and Diebold & Lopez (1995) give an overview of these models as well as their empirical evidence.Another empirical finding is the positive relation between return volatility and trading volume. A large number of contributions address this issue with theoretical as well as empirical investigations, see e.g. Foster & Viswanathan (1990), Foster & Viswanathan (1993a), Foster & Viswanathan (1993b, He & Wang (1995), Aoki (1999), Chen et al. (1999), Focardi et al. (1999 or Iori (1999) as few examples. A common approach in many of these models is to assign observed effects to the processing of private information.