Joe left his home this morning with $4 in his pocket, a long list of creditors and a feeling of irrepressible optimism. "I feel like a man in the desert who's been crawling toward an oasis," he said, "and now I'm just about to get there."

The oasis was Pimlico Race Course, though casual visitors might have difficulty perceiving its special virtues.

The track is a cavernous and, in some corners, dingy place and its appearance was not enhanced by the rain and fog today.

Pimlico's virtues are subtle ones. The track's location in the midst of a bustling Baltimore neigbothood seems to infuse it with an air of vitality. It is the only Maryland track where a horseplayer can watch the races outdoors, without sitting in a glass box, and so the Pimlico season becomes linked in the minds of betters with the coming of spring.

But for Joe and many other bettors who attended the opening of Pimlico's 76-day meeting, these qualities were secondary to another feature of the track: the bias.

Pimlico's racing strip, with its sharp turns, gives a natural edge to horses who save ground along the inside. Sometimes, in addition to this built-in bias, the inside part of the track becomes harder and faster than the outside. Then horses who race along the rail have an overwhelming advantage.

In 1977, the bias was so strong that Maryland horseplayers will never forget it. Almost every race was won by a horse with early speed and an inside post position.

In order to deal with these aberrant conditions, a handicapper had to learn only one lesson, and that was to forget everything he knew about handicapping. All he had to do was figure out who would get the early lead on the rail and let the bias take care of the rest.

For me, that season till seems like a fantasy. Simply by playing horses with the bias in their favor, I was averaging a $2,000 profit every time I passed through the Pimlico turnstiles. Winning money seemed almost indecently easy. Most other savvy horseplayers felt the same way, and they awaited the 1978 season eagerly.

It was anticlimactic. There was no bias evident. The racing was normal, with inside posts enjoying a slight edge.

So this year, nobody knew quite what to expect but Joe, for one, was optimistic. "I just had a hunch the rail was going to be strong again," he said. "I knew if it was, my problems would be over. So I borrowed $6, went to the track with $10, and decided to watch one rac and see what the track was like."

Watching the first race was a near impossibility. The track was so enshrouded in fog that the 10,539 patrons could see only a vista of solid gray. (Announcer Dick Woolley's call of the race went: "They break from the gate . . . The field of horses is bunching on the backstretch . . . They move around the turn jockeying for position . . . ")

But when the horses came into view, Cornish Verdict was leading, hugging the rail, and Sandargent was following along the rail. Nobody was moving along the outside. Encouraged by this evidence, Joe bet the speed in the second race, Music Time, and watched him pop to the rail and lead all the way.

The bias was back! The exacta numbers in the next four races were 1-2, 5-2, 3-2, 3-1, as horses with speed on the rail were winning practically everything. Joe rolled his $10 stake up to $217, and left the track not merely feeling optimistic, but knowing that future success was inevitable.

Every reasonably sophisticated horseplayer had to feel the same way. As long as the Pimlico bias exists, it is as if a money tree is blooming in northwest Baltimore.