Sample Apex HTTP Callout to Pardot API http://developer.pardot.com/

PardotAPIQueryDemo.cls

String email = '<pardot email/username>';
String password = '<pardot password>';
String userKey = '<pardot user key>';

HttpRequest req = new HttpRequest();
req.setEndpoint( 'https://pi.pardot.com/api/login/version/4' );
req.setMethod( 'POST' );
req.setBody( 'email=' + email + '&password=' + password + '$&user_key=' + userKey );

HttpResponse res = new Http().send( req );

System.debug( res );
System.debug( res.getStatus() );
System.debug( res.getBody() );

// poor man's parsing to retrieve api key without using xml/dom parser
String response = res.getBody();
Integer startIdx = response.indexOf( '<api_key>' ) + 9;
Integer endIdx = response.indexOf( '</api_key>' );
String apiKey = response.substring( startIdx, endIdx );
System.debug( apiKey );

req = new HttpRequest();
req.setEndpoint( 'https://pi.pardot.com/api/event/version/4/do/read' );
req.setMethod( 'POST' );
req.setBody( 'user_key=a911aefbf2f3ab4b70e797123ffac8a7&api_key=' + apiKey );

res = new Http().send( req );

System.debug( res );
System.debug( res.getStatus() );
System.debug( res.getBody() );