It is now rare to see periods placed after these abbreviations: 'A.M.' , but in formal writing it is still preferable to capitalize them, though the lower-case 'am' and 'pm' are now so popular they are not likely to get you into trouble.

Occasionally computer programs encourage you to write 'AM' and 'PM' without a space before them, but others will misread your data if you omit the space. The nonstandard habit of omitting the space is spreading rapidly, and should be avoided in formal writing. (Adapted from Paul Brians' Common Errors in English)